


IN 



PLANT PATHOLOGY 




Book s^ 



COPYRIGHT DEPOSm 



LABORATORY OUTLINES 

IN 

PLANT PATHOLOGY 



J\-BY 

hHh. whetzel 

II 

Professor of Plant Pathology 



LEX R. HESLER CHAS. T. GREGORY 

Assistant Professor of Plant Pathology Assistant Professor of Plant Pathology 

W. HOWARD RANKIN 

Assistant Professor of Plant Pathology 

NEW YORK STATE COLLEGE OF AGRICULTURE 
AT 

CORNELL UNIVERSITY 

ITHACA, N. Y. 

191G 



PUBLISHED BY THE AUTHORS 



SE.73I 

■ws- 



COPYRIGHT, 191 6 
H. H. WHETZELL 



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0Cr3O19l6 



W. F. Humphrey, Printer, Geneva, N. Y. 

©GI:A4453v.f-L 



PREFACE 

THESE outlines are designed solely for the purpose of most effectively 
acquainting the student with the laboratory materials presented 
in this course. They are the result of several years of experience 
and test in actual practice work. They have been frequently revised. 
It is expected that they will now be revised and reprinted at least every 
two years. 

Although the acquisition of a body of facts is an important and neces- 
sary part of the work in such a course a more vital feature is the training 
in logical methods of acquiring them. The student is urged to follow 
his outline carefully, making sure at each step, that the outline and the 
materials before him agree. The same sequence of treatment is followed 
throughout all the outlines. This sequence in procedure should be mast- 
ered promptly. The term papers will afford opportunities for determining 
how well the student has grasped the logic of this procedure. 

The grouping of diseases here presented is, we believe, an important 
step in diverting attention from the domination of systematic mycology 
in phytopathological teaching and writing, and of directing it toward 
the more logical classification and study of diseases on the basis of the 
pathological phenomena exhibited. At the same time the subgrouping 
of the diseases, according to the chief etiologic factor involved, provides 
for a point of view still generally presented in the teaching of plant path- 
ology- . 

It is not expected that all the diseases herein outlined will be covered 
in a three hours' course. The instructor will make such selections from 
the different groups as will best serve his purpose in illustrating the funda- 
mentals of the subject in the case of the students in his classes. No 
laboratory practice in the methods of control of the diseases studied is 
provided. That phase of the subject is fully treated in the course based 
upon and following this, namely, The Principles of Plant Disease Control. 

These outlines are designed specifically for the work as given at Cornell 
University and without any attempt to adapt them for use in other institu- 
tions. It is hoped, ho>vever, that teachers elsewhere may at least find 
them helpful and possibly usable in their classes. 

Acknowledgments are due Mr. Chas. Chupp, instructor in the depart- 
ment of Plant Pathology for the preparation of a number of the outlines. 
We also gratefully acknowledge the friendly advice and assistance of the 
Comstock Publishing Company in our effort to make the cost of these 
outlines to the student as reasonable as possible. 

THE AUTHORS. 



CONTENTS 



Page 

Field Trip 7 

Literature of Plant Diseases 10 



NECROTIC DISEASES 
Caused by bacteria 

Fire Blight , 15 

Streak of Sweet Peas 19 

Bean Blight 21 

Black Rot of Cnicifers 23 

Caratovorus Soft Rot 25 

Caused by fungi 

Damping-oflf of Seedlings 28 

Late Blight of Potatoes 31 

Downy Mildew of Cucurbits 34 

Sclerotinia Rot 36 

Botrytis Blight of Peonies 40 

Brown Rot of Stone-fruits 44 

Bean Anthracnose 48 

Anthracnose of Sycamores and Oaks 51 

Black Rot of Grapes 55 

Hard Rot of Gladioli 58 

Strawberry Leaf-spot 61 

Pseudopeziza Leaf-spot of Alfalfa and Clover 64 

Cercospora Leaf -spot of Beets 66 

Septoria Leaf-spot of Celery 68 

Black Spot of Roses 70 

Leaf -spot Diseases 72 

Bitter Rot of Apples 74 

Apple Blotch 77 

Leaf-cast of Conifers 80 

Black Rot Canker of Apple 82 

Endothia Canker of Chestnut 85 

PJiizoctonia Stem-rot 90 

Polyporaceous Wood-rots 94 

Ring-shake of Spruce 98 

Igniarius Heart-rot 101 

Fence Post-rot 104 

Cause, not an organism 

Stippen 107 

5 



6 

HYPOPLASTIC DISEASES 

Caused by fungi page 

Downy Mildew of Grapes 109 

Downy Mildews of the Ranunculaceae 113 

Powder}' Mildews of Florists' Crops 117 

. \ Powdery Mildew of Cereals and Grasses 120 

Apowdory Mildews of Trees and Fruits 123 

Apple Scab 128 

Ergot of Rye 132 

Onion Smut 135 

Loose Smut of Wheat _ 137 

Loose Smut of Oats 139 

Stinking Smut of Wheat 141 

Asparagus Rust 143 

Black Stem-rust of Cereals and Grasses 148 

Carnation Rust 153 

Caused by phanerogams 

Dodder 155 

Cause, not an organism 

Tobacco Mosaic 158 



METAPLASTIC DISEASES 

Caused by slime-molds 

Spongospora Scab of Potatoes 1 62 

Club-root of Crucifers 166 

Caused by bacteria 

Legume Tubercles ; 170 

Caused by fungi 

Peach Leaf-curl 173 

- Leaf -blister of Oaks 176 

Black Knot of Plums and Cherries 178 

Com Smut 181 

Hollyhock Rust 184 

Rust of Cedar and Apple 187 

"VBlister-rust of White Pine 190 

Caused by phanerogams 

-Mistletoe of Junipers 194 

Cause, not an organism 

Oedema 195 



MISCELLANEOUS 

Term-paper Subjects 196 

Glossary 205 



FIELD TEIP 

This first exercise consists of a field trip taken to points close to the 
laborator3^ The purpose is to introduce the student to a variety of plant 
diseases as they appear in the field. Attention will be directed to such 
diseases of all sorts of plants as are found. 

The student himself should seek to discover diseased plants. He will 
be expected to collect at least ten specimens of diseased plants for study 
and report. Healthy specimens for comparison should also be taken. 
(See REPORT at end of exercise.) 

For field identification of diseases one must depend largely upon 
symptoms and signs. By symptoms is meant those changes induced 
in the diseased plant which distinguish it from the healthy. Signs of 
disease are incidental or experimental evidences of disease and not the 
direct results or expressions of the diseased conditions. 

SYMPTOMS 

Symptoms are usually the expression of structural changes of one kind 
or another (histological or morphological) . Diseases in plants are of three 
general types within each of which are exhibited a considerable variety 
of symptoms. These three types of disease with some of their more 
characteristic symptoms are: — 

NECROTIC DISEASES— those in which the most striking effect on 
the host is the death of the affected tissues resulting in such symptoms as : — 

Rot — the term applied when the killed tissues become discolored 
and decayed. If the tissue in such lesions are dry and firm it is called dry 
rot; if soft and mushy, soft rot; if firm or tough but very watery or soggy, 
wet rot; if white in color, white rot; if black, black rot. The term rot, coupled 
with the name of the part or organ affected ser\^es to designate a variety of 
symptoms such as, stem-rot, bud-rot, crown-rot, collar-rot, root-rot, heart-rot, 
foot-rot. 

Blight — the term applied when there is rapid killing of the affected 
parts often accompanied by wilting and withering of the foliage. This 
term coupled with the name of the host organs gives, twig-blight, blossom- 
blight, body-blight, leaf -blight , and the Irlce. 

Wilt — the term applied where all or a portion of a plant becomes 
limp due to loss of turgor. Wilt differs from blight in that the causal 
factor does not directly affect the wilting or dying organs. The injury 
results from the activities of the causal factor usually in the vascular 
system somewhere below the wilting organs. 

Spot — the term uSed to designate necrotic areas, usually those in 
leaves, fruits and herbaceous stems. This term is used in numerous 
combinations to designate a variety of spot-symptoms as, leaf-spot, 
fruit-spot, pod-spot, and the like. 

Shot-hole — the term applied to limited necrotic spots on foliage 
where the dead tissue drops out leaving a hole. Peculiar largely to the 
foliage of peaches, plums and cherries. 

Damping-ofif — used to designate the symptom resulting from the 
rotting of the stems of seedlings at the base and the consequent falling-over 
of the tops. A form of stem-rot. 



8 ■ 

Canker — the term applied to definitely delimited necrotic lesions 
in the bark of woody plants or the cortex of herbaceous stems; smooth 
or roughened, sunken or raised. 

HYPOPLASTIC DISEAvSES— those in which the most striking effect 
on the plant is a halting in some feature of its normal growth and develop- 
ment resulting in such symptoms as: — 

Dwarfing — the term applied in those cases where the plant or organ 
does not reach normal size. Special terms like, curly-dwarf, leaf-roll, 
spindling-sprouts . and little-peach, are but disease names used to designate 
peculiar forms of dwarfing. 

Chlorosis — used to designate the failure of or insufficient development 
of chloroph}'!. Especially characteristic formiS of chlorosis have been 
designated by such names as, mosaic, calico, frenching, and yellows. 

METAPLASTIC DISEASES— those in which the most striking effect 
of the disease is the overgrowth or overdevelopment of the affected tissues 
or organs. Stimulation of growth and development beyond the normal 
results in such symptoms as :— 

Hypertrophy — the term used in its broadest sense to designate all 
abnormal overgrowths in size of diseased parts or organs. Among the 
specially designated forms of hypertrophy are, galls, knots, and tubercles, 
which are more or less globose swellings of leaves, stems, fruits or roots 
resulting from the stimulating activities of such causal factors as insects, 
fungi, bacteria and mechanical injuries. Combinations with the names of 
the plant-organs affected give the terms, crown-gall, root-gall, root-knot, and 
root-tubercles . 

Witches'-brooms — broom-like growths resulting from the dense 
fasciculation or clustering of branches due to the forcing of adventitious 
buds from or about the diseased tissues. 

Hairy root — abnormal root development of the same character and 
nature as witches'-brooms on the limbs. 

Scab — definite areas on fruits, tubers and other organs usually 
due to injury followed by abnormal cork-development resulting in a rough 
raised or sunken spot. 

Curl — a term applied usually to overgrowths of leaves resulting in 
a thickening and fluting or puffing of the diseased areas, usually accom- 
panied by abnormal coloration. 

SIGNS 

The more striking and diagnostic signs of disease are the characteristic 
fruiting structures of the pathogene. Some of these have received patho- 
logic designations, as: — 

Smut — the black powdery spore-masses developed by the so-called 
smut-fungi. Found usually in the fruiting structures of the host, although 
sometimes in leaves and stems. 

Rust — the yellow, red, brown or black powdery spore-masses of the 
rust-ftmgi usually produced in small pustules on leaves and stems. 

Mildew — the superficial mycelial or conidial structures developed 
by certain fungi on the lesions. When such growths are mealy or silky 
they are designated powdery mildews, when fluffy, downy mildews. The 
former are usually white and on the upper surface of the leaves, while the 
latter are usually grey or purplish and on the under surface. 



9 

Mould — grey or black fluffy growths, mycelial or conidial structures, 
usually accompanying necrotic lesions; sometimes used synonomously 
with mildew. 

Ooze — sticky gummy or fluid exudates from certain lesions. 

Punks — sporophores of basidiomycetous shelf -fungi whose presence 
on the surface of apparently healthy trees is evidence of heart- or sap-rot. 

Attempt to identify with one of the above, each symptom or sign of 
disease observed on the trip. Classify the signs and symptoms represented 
in the collected materials. 

The instructor in charge will explain the cause, stages of development 
and other points of interest concerning some of the diseases met with. 

REPORT 

1 . Describe and sketch the signs and symptoms of ten diseases, 
illustrating the different types observed during the trip. 



THE LITEMATUME OF PLANT DISEASES 

The literature on plant diseases is widely scattered. Abstracts of the 
current literature on the subject (American and some of the foreign) 
appear regularly in each number of the experiment station record. 
Other journals and publications in which such reviews and abstracts 
of phytopathological papers are to be found, follow: — ■ 

Phytopathology. (Official organ of the American Phytopathological 

Society.) 
Zeitschrift ftir Pflanzenkrankheiten . 
Botanisches Centralblatt. 

Centralblatt fur Bacteriologie und Parasitenkunde. 
Just's Botanische Jahresbericht. 

Index au Bulletin Bibliographique Herbdomadaire Institute Inter- 
national d' Agriculture. 
Hollrung's Jahresbericht uber das Gebeit der Pflanzenkrankheiten. 
Mycologia, formerly Journal of Mycology. 
Texts and reference books in which references to the literature of the 
subject are more or less brought together, are: — 
Duggar, B. M. Fungous Diseases of Plants. 1909. 
Stevens, F. L. The Fungi Which Cause Plant Disease. 1913. 
Sorauer, P. Handbuch der Pflanzenkrankheiten. 1909. 
Smith, E. F. Bacteria in Relation to Plant Diseases, vol. I-IIL 

1905-1911-1914. 
Massee, G. Diseases of Cultivated Plants and Trees. 1910. 

PROCEDURE IN PREPARING A BIBLIOGRAPHY 

Choose from the list of term-paper subjects, according to directions 
there given, the disease on which a term paper is to be written. 

Write in three different columns on one of the reference-sheets provided; 
(a) the names of the host-plants; (b) different names applied to the 
disease; (c) different names applied to the pathogene. 

Note: — Begin each column with the names given in the subject- 
list, adding to each; other hosts, other names of the disease and other names 
of the pathogene, discovered as the work proceeds. These will constitute 
key-words under which references to the disease in hand will be looked 
for in the indices. 

To find and copy references : — 

A. Select the latest volume of the Experiment Station Record 
available and, turning to the index, locate under key-words the references 
to the disease chosen. 

1. Write on a sheet of paper the number of the volume in hand. 

2. Record under this volum.e number, in numerical order, the 
pages on which references to the disease are abstracted. (See Information 
p. 12.) 

Suppose for example the key-words as found in the subject-list for the 

term paper are: — 

Apple Bitter Rot Glomerella cingulata 

First consult the index under apple, noting the page of every item 

that may possibly refer to the disease bitter rot, and record as directed 

above. 

10 



11 

Then consult the index under bitter rot, for you may find there 
references to the disease not Hsted under apple. 

Finally consult the index under the names of the pathogene. 

3. Examine each reference in the text carefully, and copy 
according to directions under B below, any of the references which refer 
in any way to the disease. 

B. Copy the references on the 5x8 sheets of paper provided 
using one sheet for one reference only. Begin at the very top. Write 
lengthwise of the sheet and arrange the data as follows: — 

1. Record the source of the reference in the very upper right- 
hand corner thus: — E.S.R. 18:748. (See Information on, — Source of 
reference p. 12.) 

2. Place the surname of the author, followed by his initials 
in the tipper left-hand corner on the line below that of the source of the 
reference. 

3. Directly following the author's name record in order: — 
(title) (publication) (series) (vol.) (part) (pages) (plates) (figures) (year) 

Bitter rot. Jour. Agr.Sci. 2 : 47 : 3 : 37-47, pi. 1-4, fig. 1-9. 1906. 
(See sample sheet below.) 

Note: — No matter what the arrangement and ptmctuation given 
in the source of the reference or in the original, it should be arranged and 
punctuated as above. 

When in doubt on any point, consult "Information" under the 
proper heading following the sample-sheet, page 12. 

C. Key-words. These should always appear on the reference- 
sheet. (See sample-sheet below.) First the name of the host or hosts, 

SAMPLE-SHEET 



Apple 


Bitter rot 


Grape 


Anthra,cnose 


Pear 


Ripe rot 



E.S.R. 18:874 
HASSELBRiNG, H. Bitter rot. Jour. Agr. Science. 2:47:3:37- 
47, pi. 1-4, fig. 1-9. 190(3. 

[Glom.erella cingulata] 
Glom.erella rufomaculans 
Gloeosporium fructigenutn 

Abstract from original. 

HOSTS 
Hosts listed are apple, pear, etc. p. 37. 

VARIETAL SUSCEPTIBILITY 

Most susceptible. Greening, Baldwin; least susceptible, 
Winesap, etc. p. 39. 

DISEASE 

NAMES 

Following names listed; bitter rot, anthracnose, ripe rot. 
The first most commonly applied, p. 40. 

(Continue on other sheets under the pertinent heads and 
subheads as used in the outline for the term paper.) 



12 

especially dealt with in the article should be written close to the left 
margin of the sheet and one or two spaces lower than the last line of the 
reference, in a column one above the other. 

In the same manner place the names of the disease at the middle 
of the sheet and the namics of the pathogene at the right of the sheet. 

These key-words should be for each sheet, only those which appear 
in that particular reference. If none appear in the reference (for example, 
if the name of the pathogene is entirely omitted), write the name generally 
accepted and enclose in brackets thus: — 

Apple Bitter rot [G. cingulata] 

The chief purpose of these key-words is to assist in assorting 
and identifying the references later. 

INFORMATION 

Source of reference. Having this with each reference will enable 
one to readily verify it or correct errors made in copying. It should 
include the abbreviated name of the publication, volume-nximeral (or of 
year or number as case may be), colon (:) followed by the page on which 
the reference occurs. If the article is first discovered in the original, 
indicate thus : — "Orig. ' ' 

Author. Surname, com^m^a, initials, period, is the order of the arrange- 
ment. Where there are two or more authors, arrange each name in the 
same way connecting the nam.es with "and" or its foreign equivalent, 
or with "commas" and "and" in the case of more than two authors. 
In the case of several authors one may write for example, — Stewart, F. C. 
and others, or et al. In the case of anonymous articles, write in place 
of the author's name the word, Anonymous. 

Title. The title of an article follows directly after the author's name 
and should always be in the language of the original if possible. When 
only the translation of the title is given precede it with the name of the 
original language and the word, title, in brackets, thus: — [Italian title] 
"Concerning the influence, etc." When both the original and the transla- 
tion of the title are given, the translation follows the original and is to 
be inclosed in brackets. No abbreviations of the title should be used 
except such as appear in the original. Only the first word and proper 
names in the title may be capitalized, except when the title is in German. 
The title terminates with a period. 

Name of society or organization publishing the work follows directly 
after the title, the second line beginning directly under the fourth letter 
of the author's surname. It is followed by a period; abbreviation 
allowed. 

Name of publication. Journal, Berichte, bulletin (properly abbre- 
viated) follows directly after the name of the society or organization, or 
where these are wanting, directly after the title. When the last word 
is not an abbreviation, no punctuation-mark follows. 

Volume-numerals are always to be in arabic and are to be under- 
scored with a wavy line, indicating bold-faced type in printing. The word 
volunie or its abbreviation (vol.) should not appear in a reference. 

Reports are often issued for a given year as for example the Alabama 
Agr. Exp. Sta. Report for 1896, no volume-number being given. In such 



13 

a case the year-numerals take the place of volume-numerals and the above 
wovild appear thus: — Alabama Agr. Exp. Sta. Rept. 1896:1-87. 

Number-numerals when indicating consecutively paged parts of a 
volume (as for example, vol. 27, No. 6 pp. 70-83) should not appear in 
the reference. When not paged consecutively the number-numeral 
appears between volume-numeral and page-numeral. 

Bulletins, circulars, memoirs, leaflets and the like are usually numbered 
and are not considered as constituting consecutively paged numbers of 
a volume. The numerals appear therefore in place of the volume-numerals, 
but are not to be underscored. The proper abbreviation for the word 
Bulletin is Bui. with a capital B. Where qualified, as for example. Techni- 
cal bulletin, it is abbreviated but not capitalized. 

Series, Abteilungen. If a periodical is issued in two or more separate 
series the series-numeral precedes the volume-numeral, separated from it 
by a colon and is no^ to be underscored, as: — 3:46:23-87. Sometimes 
the series is indicated thus: — Science n. s. 42: 47-56. The n.s. = New 
Series. 

Parts. If parts of a volume are paged separately, the part-numeral 
follows the volume-numeral and is separated from it bv a colon: — thus, 
82:2:241-256. 

Pages. The page-numerals when preceded by volume- or number- 
numerals and a colon are to be cited inclusively, i.e. first and last pages 
separated by a dash thus: — 47-83, and followed by a period. 

Tables, plates and figures follow directly after the pages, properly 
abbreviated, followed by the proper numerals in arabic. 

Year. This must be the actual year of pubh cation. It may not 
always be determined with certainty except by consulting the original. 
Exercise care on this point and leave the space blank until the original 
can be consulted, if not absolutely sure of it from the reference. 

Books. In cases where no volume is given, cite the pages thus : — p. 27 
or p. 1-441. 

Citation from books. Following the date, give the title of the chapter 
or paragraph, especially referred to, then. In, then the title of the book 
followed by the letter p. to indicate page, then the page number and finally 
the date thus: — 

Ellis, J. B. Dothidea pomigena. Schw. In The North American 
Pyrenomycetes. p. 605. 1892. 

Government publications are always to be cited in the following form; — 
U. S. Agr. Dept. Yearbook; or U. S. Plant Ind. Bur. Bui. 37: 

State Experiment Station publications are always to be cited thus: — 
Alabama Agr. Exp. Sta. Bui.; or New York (Geneva) Agr. Exp. Sta. 
Rept.; Cornell Univ. Agr. Exp. Sta. Bui. 237:46-91. 

Note: — For further details and examples of correct citation see 
pamphlet "Notes for guidance of author's." A copy may be checked from 
the departmental library. 

PREPARATION OF AN ABSTRACT OF AN ARTICLE 

The abstract should always be made from the original, unless for some 
good reason this is impossible. Do not copy the abstract of the article 
as it appears in the Experiment vStation Record or elsewhere. 



14 

Begin the abstract on the reference-sheet directly beneath the key- 
words. Use both sides of the paper or use additional sheets or both if 
desired. 

One should abstract as he reads, assorting and entering data under 
the proper head as indicated in the outline for writing term paper. (See 
term — paper outline and sample-sheet in this outline p. 11.) 

Follow each entry of data with the number of the page on which it 
was found in the original. This will enable one, when writing the 
term paper to readily cite the source of any statement made. 

The abstract should be full enough that it will not usually be necessary 
to refer again to the original in preparing the term paper. 

REPORT 

Hand in the bibliography of the subject chosen, arrange as follows: — 

1. The references, each on a separate sheet. Arrange the 
sheets in the manilla folder provided, in the order of importance of the 
references, — ^the most important first. 

2. One or more of the most important references carefully 
abstracted as above described. 

3. The folder labeled thus: — 



(Name of host) (Name of disease) Bibl. 



(Name of student). 



fECEOTIC DISEASE! 



FKKE BLIGHT 

This is the most common and best known bacterial disease of plants 
occurring in this cotmtr}\ It affects apples, pears, quinces and occasion- 
ally plums, apricots and a few ornamental and wild plants related to the 
apple family. 

SYMPTOMS 

The symptoms of this disease will be studied in the order in which 
they manifest themselves during the season on different parts of the tree, 
beginning with the appearance of the cankers in the spring. 

Hold-over cankers. These are the sources of inoculum for the first 
infection of the blossomis in the spring. Study the typical cankers on 
the limbs of apple and pear trees provided and observe: — 

1 . The smooth, mi ore or less sunken area in the bark, — the canker; 
its margin sharply defined by a definite crack. In cankers in which 
the pathogene is active this margin is not sharply defined. (»See illustra- 
tion specimens if available; photograph 1; Cornell Bui. 272, fig. 16, or 
329, fig. 114.) 

2. The margin. Note that it is irregular, the crack being 
formed by the drying away of the diseased tissue from the healthy when 
the active progress of the pathogene is suddenly checked. Dry or cold 
weather may thus check the enlargement of the canker. These specimens 
were collected in the autumn or winter. 

3. The surface of the canker. Note that it is smooth, seldom 
roughened or wrinkled. It is often checked at the margin by drying. 
Compare with the healthy bark in this respect. Locate the lenticles. 
What is their structure and function? 

Make a V-shaped cut across the margin of the canker and determine : — 

4. How deeply the disease penetrates. What tissues are 
affected ? 

Make a drawing of the canker studied. Label fully. These cankers 
are formed during the summer and early autumn and in many of 
them the bacteria pass the winter dormant, or only slightly active 
in the partially living tissues along the margin. With the increased 
temperature and the beginning of growth-activities in the spring, these 
bacteria become active, work rapidly into the -adjoining healthy tissue, 
increase the area of the canker and ooze out through the lenticles to the 
surface in sticky, milky drops. Study photograph 1; Cornell Bui. 272, 
fig. 16, or 329, fig. 114. observe: — 

5. That the advancing margin of the canker is not distinctly 
evident here. The diseased area covers nearly all the surface shown in the 
illustration, except on the extreme left. 

6. The large viscid milky drops ooz'ng out and running down the 
limb; above to the left, two small globules just oozing out from the lenti- 
cles. Read Cornell Bui. 272:40-41, also Ontario Bui. 176: 15-23. 

15 



16 

Make a drawing from photograph 1; Cornell Bui. 272, fig. 16, or 329, 
fig. 114. Label fully. 

Blossom-blight. Bees and flies visit these active cankers in the spring, 
to feed on the exuding sap and then visit the opening blossoms where they 
leave behind them some of the bacteria which adhered to their bodies. 
Here in the nectar and in the injuries made by the insects' claws in the tender 
tissue of the flower, the bacteria multiply rapidly killing the blossoms. 
Study the specimens provided; Ontario Bui. 176, frontispiece; Cornell Bui. 
329, fig. 112; or photograph 2. observe: — 

7. The dead and blackened flowers. The leaves of the spur are 
also dead and brown. The bacteria have spread down the pedicles into 
the spur. These dead and blackened blossom-spurs are usually the first 
striking evidence of the disease in the spring. The oozing cankers are 
usually overlooked. Make an outline drawling of a blighted blossom- 
spur. 

Fruit-blight. Frequently only one blossom on a spur is infected and 
by the time the bacteria have killed it and worked their way down the 
pedicle to the spur itself, the uninfected blossoms have developed fruit 
of a considerable size. From the spur the bacteria now work into the base 
of these fruit-pedicles and by way of them into the growing fruit. Study 
photograph 3; Ontario Bui. 176, fig. 15-19; or illustration specimen. 
OBSERVE : — 

8. The blackened pedicle and discolored lower half of the fruit. 
A fruit affected in this way usually shrivels and drops from the tree. It 
may cling to the twig as a blackened and shriveled mummy. The loss 
from fruit-blight is somiCtim-es heavy. The curculio and aphids fre- 
quently introduce the bacteria into the fruit through their pimctures. 
The pathogene does not always enter the fruit by way of the pedicle. 
Note that the leaves of the spur are also dead and shriveling. In rainy, 
muggy weather the bacteria ooze from these blighted fruits and blossoms 
in sticky drops as they do from the hold-over cankers. Study photographs 
4a and 4b. observe: — 

9. The discolored and slightly sunken tissues about the base of 
the stem and extending toward the blossom end. The sticky, milky 
drops oozing from the diseased area. ■ A large one on the pedicle. Make 
DRAWINGS (from specimens or photographs) showing these different phases 
of fruit-blight. 

Twig-blight. The bacteria from the diseased blossoms and fruits 
are carried by sucking insects to the tips of the growing shoots and water- 
sprouts and are there introduced through the wounds or punctures made 
by the insects, into the tender succulent tissues. Here they multiply 
rapidly killing the shoot, causing the form of the disease known as twig- 
blight. Blighted and healthy twigs (fresh and preserved) are provided. 
Examine and observe : — 

10. The contrast between the diseased and healthy portions 
of both the twigs and leaves. You may be able to find the dried ooze. 
(See illustration specimen.) sketch to show contrast between diseased 
and healthy twigs. 

11. That in some of the specimens the dormant buds in 
the axils of the leaves just below the blighted portion have been pre- 
maturely forced. Explain this. (See illustration specimen or photograph 6.) 



17 

Origin of the cankers. Examine the specimens of cankers again 
verv careftdly and observe : — 

12. That the cankers almost always surround the base of the 
spur, twig or watersprout, or that there is at least one or more within 
the area. Sometimes it is a fruit-sptir, the bacteria had entered by way 
of the blossoms; more frequently it is a water-sprout, the tender suc- 
ctdent growth of which is ver\^ favorable for the rapid development 
of the disease. The bacteria kill the water-sprout do\yn to the trunk 
or limb and spread out into the bark often for a considerable distance 
around the base of the sprout. (See illustration specimens and photograph 
8.) If the previous drawing of the canker does not show the dead spur 
or water-sprout, draw to show it. 

Cankers occasionally originate in other ways. Sometimes the pruning- 
knife carries the inocultmi. Cankers may start from insect- wounds 
in the bark. (See Ontario Bui. 176:33-38.) 

ETIOLOGY 

The organism that causes this disease is Bacillus amylovorus (Burrill) 
Trevisan. 

Life-history. Laboratory studies in the life-history of this pathogene, 
aside from the facts brought out in the above study of symptoms, are 
necessarily limited to a short study of the morphology' of the bacillus. 
Before proceeding with the following observations the student should 
have read carefully that portion of the text* dealing with the life-history 
of the organism. 

If pure cultures are available, examine them carefully and observe :-- 

13. The character of the growth on the surface of the solid 
media. The effect on the bouillon. Why?" How long has it taken to 
produce this efTect? What does that indicate as to the rapidity of multi- 
phcation? Make a mount from a pure culture and before covering intro- 
duce a bee's foot, observe: — 

14. The size and motihty of the organism; cell-unions. Com- 
pare it in size with the claw of the bee's foot. It may now be understood 
how a bee or fly might carry thousands of these on its feet, draw the 
bee's claw and some bacteria beside it. Maintain relative proportions. 

Study photograph 9 or the stained mount under the demonstration 
microscope and make out : — 

15. The ciHa; their number, length and distribution over the 
thallus. They are peritrichic. It is this character that places the organism 
in the genus BaciUus. 

If fresh material is available crush a bit of the most recently affected 
tissue in water on a slide, cover and try to demonstrate: — 

16. The living bacteria. How do they compare in size, form and 
abundance with those from the pure culture? Are they motile? 

Pathological Histology. Cross-sections of fresh growing twigs show- 
ing both healthy and diseased tissues are to be studied (supplemented with 
stained section provided). Examine the healthy portion and observe: — 

17. The reddish brown outer coat, made up of the epidermis 
and the several layers of cork-cells beneath. 



*This refers to anv text which is available. 



18 

18. The large globose or oval cortex-cells, closely packed next 
the cork-layer but with looser arrangement toward the wood. Their 
color and contents. 

19. Just beneath and partly surrounded by the cortex-cells, 
bundles of white densely packed sclerenchyma-fibers (bast). 

20. Toward the center in the following order, the phloem, 
cambium, and xylem. 

21. At the center of the cross-section, — the pith; the medullary 
rays distributed radially in the vascular cylinder. 

Study the diseased portion of the twig, comparing the blighted tissues 
with the healthy tissues already examined, observe: — - 

22. The affected area. Which tissues are involved? How 
recognized? Are all the cells within the diseased region killed? 

23. The effect of the pathogene on the epidermis, cork, cortex, 
sclerenchyma-fibers, medullary rays, phloem, cambium, xylem and pith. 
Compare with the healthy condition of the cells as to form, size, color and 
contents. 

24. The relatively large cavities scattered throughout the affected 
portion. How may these be accounted for? 

25. The stratified appearance just outside of the phloem. To 
what due? 

26. The apparent absence of the bacillus. Why not evident? 
drawings:- — (a) Make a diagrammatic drawing to show the healthy 

and diseased tissues, (b) A much larger and detailed drawing of a portion 
of a cross-section of the affected twig. Make the drawing include both 
healthy and diseased bark. 

Study closely the margin of the lesion. In case the section was taken 
from a twig in which the pathogene was inactive the diseased and healthy 
tissues will be separated by a cork-layer. If the pathogene was active 
no cork-layer will have yet been formed. Determine which was the 
condition in the twig from which the section provided was cut. In the apex 
of growing twigs, the parasite may invade the xylem-ducts and thus 
migrate some distance in them down the twig. 

REPORT 

1. In working out methods of control, of what importance are 
the following facts about fire-blight : — 

a. It occurs only in North America. 

b. The bacteria causing the disease pass the winter in hold- 
over cankers in any of its numerous hosts. 

c. The bacteria get into the host through wounds. 

d. The chief agents of inoculation are certain insects. 

e. The bacteria are usually introduced into the young and 
growing parts of the host where in these succulent tissues they multiply 
and develop the disease very rapidly. 



STREAK OF SWEET PEAS 

This bacterial disease affects not only sweet peas but many other 
legumes including clover. 

SYMPTOMS 

Examine the specimens provided (fresh, dry, or in liquid), observe : — 

On the stems. 

1. The streak-like lesions; color, extent, part of stem affected. 
Look for young isolated lesions; old lesions. How do they differ as to 
extent and color, draw to show variation in stem-lesions. Label to 
indicate colors. 

On the leaves and tendrils. 

2. The similarity of the lesions to those on the stem. Find a 
stem-lesion that has extended along the petiole into the blade at its base. 
Note the reddening of the affected veins and the dead blade-tissues between. 

DRAW. 

3. Locate isolated lesions or spots in the leaf. Study carefvdly. 
DRAW and label fully as to markings and colors. 

On the flower. 

4. The character of the lesions on the petals. Especially striking 
in dark flowers, draw. 

ETIOLOGY 

The cause of this disease, long unknown, has recently been shown to be 
Bacillus lathryi Manns and Taubenhaus. This pathogene appears to 
be widely distributed in England and United States. 

Life-history. Little is certainly known of the habits of this parasite. 
It is supposed to pass the winter in the soil and decaying plant parts which 
were killed the previous year. 

The Primary Cycle is initiated about the time the peas begin to 
blossom. 

Pathogenesis. The bacteria in the soil are splashed by the rain 
upon the plants and gain entrance to the tissue through the stomata. 
Peel the epidermis from the stem of a healthy sweet pea. Mount outside 
up and observe : — 

5. The epidermal cells; their shape and arrangement. The 
stomata; numbers and structure, draw. These stomata are the infec- 
tion-courts. The bacteria in the moisture on the stem pass through the 
stomatal opening into the substomatal cavity. Here they multiply and 
by their toxic secretions kill the adjoining cortical cells. The disorganized 
juices of these cells diffuse into the substomatal cavity affording food for 
further growth and multiplication of the pathogene. 

Make sections through a young lesion on the stem (or use prepared 
slides). Examine and if possible detect: — 

6. The bacteria in the tissues. Are they between or in the cells? 
draw to show the relation of the bacteria to the cells. 

Saprogenesis. Whether the pathogene is able to live and multi- 
ply in the soil is an open question. It is, however, readily cultivated 

19 



20 

on nutrient media where it can be better studied than in the tissues of the 
host. Examine the plate-colonies of Bacillus lathryi. observe: — 

7. Their form, size, color and consistency, draw. 

Moimt a bit of one of the colonies in a drop of water. Cover and 
OBSERVE : — 

8. The size, shape, motility and cell-unions of the bacteria. 
DRAW; and COPY also from Delaware Bui. 108:22, fig. 1. 

Secondary Cycles originate from bacteria oozing from the primary 
lesions and spattered by rain to nearby leaves and stems. They do not 
differ in any other way from the primary. 

Pathological Histology. Study the prepared sections (cross- and longi- 
sections) through lesions on the stem, observe: — 

9. That at first the bacteria occupy the intercellular spaces, 
adjacent to the substomatal cavity. 

10. That these soon break down the cell-walls and invade 
the cell-cavities. (See Deleware Bui. 108, pi. I.) To what is the color 
in the lesion due? 

11. That not all the tissues of the stem are invaded. Determine 
which. Show this in a drawing of the section through the stem-lesion. 

Pathogenicity Studies. Under the direction of the instructor the 
student will, if fresh material is available, make isolations from stem- 
lesions on nutrient agar or potato-agar in a petri-dish. Set the petri-dish 
away until the next exercise at which time compare with the pure culture 
studied above. 

If growing sweet peas and clover are available, inoculate under the 
direction of the instructor, with these pure cultures of the pathogene. 
Examine the plants after two days and again later for evidences of infection. 
Record in your notes the length of the incubation-period. 

REPORT 

1. Describe fully how the pathogene was isolated, character of its 
growth in the media used, how the inoculations were made and the results. 



BEAN BLIGHT 

This is the common bacterial disease of beans. It apparently affects 
all varieties of Phaseolus vulgaris L., the field- and garden-bean, as well 
as the Lima bean, Phaseolus lunatus L. 

SYMPTOMS 
This disease affects all parts of the host above the ground, leaves, 
stems, pods and seeds. 

On the leaves. Examine the leaves provided and observe : — 

1. The location, size, color and general appearance of the lesions 
on both sides of the leaf. 

2. The zonate character of the spots. How do you account 
for each zone, especially the opaque, green water-soaked zone which shows 
best when the leaf is held to the light; also the pale green outermost zone? 

Draw to show these characters and indicate fully in the labeling, the 
color and appearance. 

On the stems. The affected stems show no distinct lesions since only 
the vascular system, as a rule, is usually involved. Diseased stems finally 
shrivel following the wilting of the leaves. This symptom is not common. 

On the pods. Examine the pods provided and observe : — 

3. The location, size, color and water-soaked appearance of 
the lesions. Pod-lesions are sometimes red-bordered. (See illustration 
specimens.) 

4. Depth to which the lesions penetrate. Determine by cutting 
across the pod through a lesion. Draw to show form and depth of the 
lesion. Label fully as to color and character. 

On the seed. Examine the lesion with a hand-lens and observe: — 

5. The discolored, roughened surface. To what is this due? 
What relation do the lesions on the seed bear to the pod-lesions ? Make a 
DRAWING showing the character of the lesions on the seed. 

Examine the passe-partouts provided and compare the symptoms of 
this disease with those of the anthracnose, a fungous disease of the bean 
which is also very prevalent. Make drawings of a pod, seed and leaf 
affected with anthracnose, shov^dng how the symptoms of that disease 
differ from those of the bean blight. 

ETIOLOGY 

The cause of this disease is Bacterium Phaseoli E. F. Smith { = Pseudo- 
monas Phaseoli EPS.), a monotrichic bacterium which produces yellow 
colonies on agar. 

Life-history. This organism is a restricted parasite, normally passing 
its entire life within the bean. It probably has no true saprogenic phase 
in its life-history. 

The Primary Cycle is initiated in the spring from the cotyledons 
of diseased seedlings. 

Pathogenesis . Diseased seed have been soaked a short time in water. 
Peal the seed-coat from the lesion. Make two mounts, one from the 
diseased seed-coat and one from the diseased cotyledon, chopping and 
crushing the tissues, observe : — 

6. The minute, short rod-shaped bacteria to be detected only 
with the high-power. Are they inside or outside the cells? draw. 

21 



22 

The bacteria in these cotyledons are carried by insects or, oozing out 
upon the surface, are splashed by rain to the leaves. The insects introduce 
the pathogene into the wounds which they make or the bacteria may 
enter through the stomata, thus initiating primary infection. The primary 
infections which are seldom to be observed, exhibit the same type of leaf- 
lesion as the secondary infections. The primary cycle is probably never 
completed, the bacteria in the primary lesions dying with the disintegration 
of the leaves. 

The Secondary Cycle infections are far more numerous and therefore 
are more destructive than the primary. They are the ones most commonly 
observed. They are repeated frequently throughout the season, initiated 
at first from the primary and later from earlier secondary infections. 

Pathogenesis. The pathogene is carried by insects or rain from 
the primary lesions to healthy leaves and pods. Examine several of 
the leaf -lesions carefully to see if you can locate : — 

7. The point at which the bacteria were introduced. Was the 
inoculating agent a biting or sucking insect? Was the lesion a result of 
stomatal invasion? 

With a scalpel, chop in a drop of potassium hydroxide (do not crush) 
a bit of leaf -tissue from the outer opaque green zone of the lesion. Cover 
carefulty and under low-power observe: — 

8. The bacteria oozing from the cut ends of the veins. Make a 
DRAWING of a vein with oozing bacteria. 

Examine several deep pod-lesions, cutting across the pod through the 
lesion or shelling out the beans. The bacteria in the pod invade the 
seed, where, with the ripening of the seed, they become dormant, thus 
completing the secondary cycle. Can evidences of invasion of the seed 
beneath the lesion be detected ? 

Saprogenesis. Although there is no evidence that this pathogene 
lives and multiplies outside the living tissues of the bean, some investi- 
gators hold that it may live over winter in the stems and leaves on the 
ground (Ontario Bui. 136:12.). It will, however, grow readily on culture 
media. Examine the plate- and tube-cultures and observe: — 

9. The color, form and consistency of both the buried and 
surface-growing colonies; effect on the bouillon-cultures, draw and label 
to indicate color and other characters. 

Make a mount from one of the surface-colonies and compare : — 

10. The bacteria with those studied from the tissues as to size, 
shape, cell-unions and motility, draw. 

For fiagella see Cornell Bui. 239 : 21 1 . copy . 

Pathological Histology. Examine the sections (freehand or prepared) 
of diseased pods, stems, or leaves, observe : — 

11. Which tissues are chiefly invaded in the organ examined. 

12. The relation of the bacteria to the cells; within or between? 

13. The effect on the cells; on the middle lamella and protoplast. 
Make drawings necessary to bring out these points. 

REPORT 

1. Detail the life-history of Bacterium Phaseoli EPS., describing 
the symptoms resulting from its activities in the different organs of the 
host. Follow the sequence and indicate the headings, used in this outline. 



BLACK MOT OF CRUCIFEMS 

This is a very dCvStructive bacterial disease of cabbage and cauliflower. 
It also affects kale, radishes, rape, turnips and mustard. 

SYMPTOMS 

This disease is detected chiefly by the lesions on the leaves, by the 
effects on the plant as a whole and on the vascular system of the mid- 
ribs and the stalk. 

On the leaves. Examine the cabbage or cauliflower leaves provided 
and OBSERVE : — 

1. The large irregular dead or dying areas in the blade. The 
yellowish or light-brown color of the lesions. 

2. Their location in the leaf. What does this indicate with 
respect to the point of initial invasion? 

3. That when held to the light the veins of the affected area 
are black. (See in N. Y. (Geneva) Bui. 232, pi. I and II; or photograph 1.) 

4. In fresh material, if available, the difference in texture of the 
diseased and healthy parts of the leaf. 

DRAW to show the character of the leaf-lesions. 

On the plant in general. Study the diseased heads provided, ob- 
serve : — 

5. The long bare stalk and loose dwarfed head often reduced 
to a mere tuft of leaves (photograph 2) . 

6. The stubs or leaf -scars on the lower part of the stem where 
the leaves have died and dropped away. 

7. That in some heads the leaves have dropped away from one 
side only. 

8. That diseased seedlings are dwarfed and fail to head. They 
often wilt and die early. (See N. Y. (Geneva) Bui. 251, pi. I.) 

In Smith's Bacteria in Relation to Plant Diseases 2, pi. 18, is shown a 
peculiar effect due to slow killing of the margins of the leaves. Note 
careftdly. 

Make drawings to show these characteristic effects on the plant as a 
whole. 

In the vascular system. Examine the leaf-scars on the diseased stalks. 
observe : — 

9. The black points, — the ends of diseased vasctdar bundles. 
This is a typical vascular disease. Break a leaf from the diseased stem, 
cut across the petiole and note :— - 

10. That here some of the bundles are also black. Split the 
petiole and midrib lengthwise. How far does the blackening extend.? 

Make transverse and longitudinal cuts through the diseased stalks. 
observe : — 

11. In the vascular cylinder, the blackened strands which here 
and there curve out to the margin. Explain. 

Make a drawing to show the stem with blackened strands both in 
cross- and longisection. 

ETIOLOGY 

The causal organism of black rot is Bacterium campestre (Pammel) 
Erwin F. Smith. It is related to the bean blight pathogene. Bacterium 

23 



24 

Phaseoli EFS., forming like that bacterium, yellow colonies on agar. It 
is also monotrichic. 

Life-history. This pathogene passes the winter either in the debris 
of the host in the soil or on the seed. 

The Primary Cycle begins in the seedlings in the seed-bed or in the 
plants after they are set in the field. 

Pathogenesis. If the seed-plants have been diseased the seed 
may become contaminated with the pathogene when thrashed. On the 
oily coat of the seed, the bacteria may remain alive for a year or more. 
Examine the agar-plates on which contaminated cabbage seed have been 
sown, observe: — 

12. The yellow colonies developing about some of the seed. 
DRAW. With your needle remove a bit from a colony to a small drop of 
water on a slide. Cover, examine and make out: — 

13. The short rod-shaped bacteria .swarming in the mount. 
Just how the pathogene on the seed gets into the seedling is not known, 
but it probably enters through the young rootlets, draw. 

The inoculum in the soil in the field is carried to the leaves of the grow- 
ing plants by snails, insects and probably by the wind. 

Examine the cabbage plant under the bell-jar. Here the conditions 
of humid weather are reproduced, observe : — 

14. The beady drops of water along the leaf-margins. This 
water is extruded from the water-pores at the ends of the veins, draw. 
In these drops the bacteria multiply and enter through the pore into the 
cavity at the end of the vascular bundle. Here they multiply rapidly, 
penetrate into the xylem-vessels and, traveling downward, kill and dis- 
organize the vascular tissues causing them to blacken. The pathogene 
thus reaches the stalk by way of the leaf -veins. 

Sapr agenesis. It is not certainly known whether the organism 
multiplies in the dead and rotting leaf -tissues and in the soil; probably 
not to any great extent. It grows readily, however, in culture media 
as has been observ^ed, forming characteristic yellow colonies. 

Secondary Cycles are initiated by bacteria carried by gnawing or 
sucking insects which have fed on diseased tissues. In this way the 
pathogene is carried from plant to plant and epiphytotics often result. 

Pathological Histology. vStudy the prepared cross-sections and ob- 
serve : — 

15. The deeply stained diseased vascular bundles. What 
tissues of the bundle are affected? 

16. The relation of the pathogene to the cells; inter- or intra- 
cellular? 

17. The numberless bacteria filling the spiral and annular 
vessels. Study the effects of the pathogene on the structure of these 
vessels as shown both in the cross- and longisections. 

Make drawings to show these effects on the tissues. 

REPORT 

1. Describe fully the ways in which Bacterium campestre (Pam.) 
EFS. may possibly hibernate and show how these determine the control 
measures to be employed. 



CARATOVOMUS SOFT MOT 

This is the best known and most common soft rot disease of plants. It 
affects a great variety of hosts in each of which it produces more or less 
characteristic and peculiar symptoms. 

SYMPTOMS 

In carrots or turnips. It was in the fleshy roots of the carrot that this 
disease was first carefully studied. Examine the diseased roots provided 
and OBSERVE : — 

1. The soft mushy character of the rotted tissues. Crush a bit 
of it between the thumb and finger. 

2. The fetid odor of putrefaction. 

3. That the rot is largely confined to the root. 
Cut through the lesion and note : — 

4. The variations in color and firmness of the diseased tissue 
from the center of the rotted mass toward the healthy tissue. 

DRAW to show these characters. 

The rotting of the roots in the field is accompanied in its advanced 
stages by the sudden wilting of the leaves. 

In cabbage and cauliflower. In these hosts the soft rot frequently 
follows the black rot and causes in cabbage the so-called "stump-rot." 
Study the diseased heads provided and observe : — 

5. That the rotting is largely in the base of the fleshy leaves (or 
in the fleshy head of the cauliflower). 

6. That the cabbage head is thus loosened from the stalk from 
which it may be readily lifted leaving a slimy rotten "stump." 

DRAW to show these symptoms. 

7. The soft mushy character of the rotted tissues; color? 

8. The fetid odor of the decomposing tissues. 

Split one of the partially rotted mid-ribs of the leaf or a diseased 
branch of the cauliflower head. Examine and note : — 

9. The differences in color and consistency of the diseased 
tissues as the healthy tissue is approached. 

In iris. This disease is very destructive to the iris both in Europe and 
America. Examine the diseased leaves provided and observe: — 

10. The soft rotted basal portion. Crush a bit of it between 
the fingers. If fresh material is available, split a healthy and a diseased 
leaf and compare as to color and odor. 

11. That the epidermis of the diseased portion is more or less 
intact forming a rather firm covering for the rotten mass within. 

12. The upper advancing portion of the lesion, firmer than below 
but dark and water-soaked in appearance, sketch external and internal 
view. 

13. That the tips and margins of some of the leaves rotted at the 
base are dying and drying out. How may this be explained? 

The most striking field-symptoms are the falling-over on the ground 
of the outer affected leaves or the shriveling and browning of the tips of 
the leaves of the cluster. This latter symptom is, however, easily con- 
fused with the dying of the tips due to a fungous spot-disease common on 
iris. (See demonstration specimen.) 

25 



26 

SKETCH to show field symptoms. (See demonstration material.) 

14. Diseased rhizomes if available. Make a similar compara- 
tive study and draw. 

ETIOLOGY 
The pathogene responsible for this soft rot in such a variety of hosts 
is Bacillus caratovorus Jones. It was first described from carrots. 

Life-history. This organism doubtless hibernates in host-debris or 
in the soil itself. 

The Primary Cycle is initiated when the particular host concerned 
is in the best stage of its development for the attack. Succulent tissue 
is a requisite. The iris is iisually affected very early in the spring while 
carrots, turnips, cabbage and cauliflower are first attacked late in summer 
or autumn. 

Pathogenesis. The source of the primary inoculum is doubtless the 
soil or rotting host-tissue in the soil. The bacteria are carried to the roots 
or rhizomes by snails, insects and probably flowing water. They can enter 
apparently only through wounds. Once within susceptible tissue with 
abundant moisture present they multiply rapidly and cause the disintegra- 
tion of the parenchyma. 

Remove a bit of the rotten tissue to a drop of water on a slide, cover 
and examine for the bacteria, observe : — 

15. Their abundance, and relation to the cells of the host. 
Are they within or without the cells; motile or non-motile; single or 
in threads? Are spores present ? draw. 

16. The photomicrograph showing the flagellate stage of the 
pathogene. How many flagella has each bacillus and how are they 
arranged? draw. 

Saprogenesis. This organism is doubtless able to multiply in 
host-debris in the soil so that virulent active inoculum is always present 
where the pathogene has flourished the previous season. It grows readily 
on culture-media. 

Examine the pure culture provided, sketch and describe: — 

17. A typical colony as to form, color, consistency and odor. 
Make a mount from one of the colonies and compare the bacteria 

with those from the tissues as to the points indicated in No. 15. 

Secondary Cycles are initiated from, and develop in the same man- 
ner as the primary cycles. 

Pathological Histology. Make several thin longisections through a 
smaU piece of the healthy organ (root, leaf -base or rhizome), identify: — ■ 

18. The various tissues present. The cell-contents and organs 
in each. Make a drawing detailing a portion of the section, a few cells 
wide, from epidermis toward the center. Show with especial care the inter- 
cellular spaces and contiguous walls of the cells. 

Remove the epidermis from over the soft portion of a lesion. With a 
needle or point of a scalpel remove a bit of the rotten tissue to a drop of 
water on a slide, cover and gently press on the cover. Study and observe : — 

19. That the individual cells are separated from each other 
but are still intact. Identify the kinds of cells seen with those in the healthy 
leaf. Compare their shape with corresponding healthy cells. 

20. The disorganized and plasmolized character of their con- 
tents. What organs of the cell can still be identified? 



27 

DRAW to show these points and the relation of the bacteria to the cells. 
Examine the prepared longi sections through the advancing margin 
of the lesion and see if it can be determined : — 

21. Just where the diseased cells leave off and healthy ones 
begin. On what characters are the conclusions based? What tissues 
are affected? 

22. How the cells are separated from one another. Relation 
of the bacteria to the separating cells. 

DRAW to show the cell-structures, dissolving middle-lamella and bacteria 
at the advancing margin of the lesion. 

Pathogenicity studies. To give the student some idea of the rapidity 
with which the bacteria destroy tissue, a simple inoculation experiment 
may be made as follows : — 

(a) Cut three short pieces of healthy leaves of iris, cabbage or cauliflower 
and three thin slices of roots of carrots or turnips and place them in clean 
petri-dishes on wet filter-paper, (b) Flame a needle and puncture one 
piece of each; these constitute the checks, (c) Flame the needle and 
when cool, draw the point through the pure culture and then puncture 
a second piece of each, (d) Flame the needle and when cool, draw the 
point through the soft diseased tissue and puncture the remaining pieces. 
Label carefully and set away until next exercise when they should be exam- 
ined. Record by sketch or description the results of the experiments. 

The instructor will demonstrate the method of isolating the pathogene 
to get it in pure culture. 

REPORT 

1. Describe and explain in detail the pathological effects of 
Bacillus caratovorus Jones on susceptible plant-tissues. 



DAMPING=OFF OF SEEDLINGS 

The name damping-off is applied to a type of disease affecting 
seedlings rather than to any one specific disease. Many species of quite 
different groups of fungi are known to cause damping-off, but as here 
used the name will be restricted to the diseases produced by certain 
phycomycetous fungi. 

SYMPTOMS 

The most striking symptom and the one which gives to this type of 
seedling-disease the name damping-off, is the peculiar effect of a necrotic 
lesion in the stem at or near the surface of the ground. Often the 
lesion originates just below the surface of the soil. A wilting and then a 
disintegration of the tissues affected, results and the plantlet falls over and 
dies. Root-rot may accompany damping-off and may prove fatal except 
where adventitious roots are put out above the lesion and remain healthy. 

Study the damped-off seedlings provided and photographs 1, 2, 3, 4 
and 5. observe : — 

1. The withering of the stem-tissues at or near the surface of the 
soil. 

2. The sharp angle made by the fallen stem. While affected 
plants usually fall over, they may remain erect and wilt if the vascular 
tissues are siiiffcientl};^ rigid. 

3. The dying of all the plants in affected areas. These damped- 
off spots are quite characteristic of infested seed-beds. 

DRAW to show these characteristic symptoms. 

ETIOLOGY 

A rather large number of fungi are now known which produce the above 
symptoms in seedlings. (See text.) As indicated above, only phycomyce- 
tous forms will be here considered. Of these it is one of two species that 
is usually responsible, either Pythium de Baryanum Hesse or Phytoph- 
thora Cactorum (Cohn and Lebert) Schroeter. These two species are 
very much alike in structure and habits so that the following outline 
should serve for either. 

Life-history. These pathogenes exhibit in their life-history both primary 
and secondary cycles, with a short period of pathogenic activity and a well- 
marked saprogenic phase, especially in the case of P. de Baryanum. 

The Primary Cycle is initiated at or shortly after the germination 
of the host. Sometimes the seedlings are killed before they get above 
ground. 

Pathogenesis. Both fungi pass their saprogenic existence in the 
soil, probably always in association with host-debris. Both produce 
resting sexual spores, — the oospores, which carry them through the winter. 
These germinate and produce the inoculum for the primary cycle. Motmt 
and examine some of the mature oospores provided (in pure cultures). 
OBSERVE : — 

4. The form, size and color of the sexual bodies. 

5. The old oogonial sac, the outer covering enclosing the single 
oospore. The thick smooth wall of the oospore and its densely granular, 
oily contents, draw. 

28 



29 

Lying in the soil near the seedHng, these oospores germinate (in the case 
of P. Cactorum at least) by sending forth a germtube at the tip of which is 
produced one or more conidia. Study Cornell Bui. 363, fig. 18. copy 
one or more of the drawings. 

These conidia germinate either by swarmspore-formation or by germ- 
tubes and infect the tender tissues of the seedlings. (See Cornell Bui. 363, 
fig. 16-17.) Conidial germination will be studied later. 

The mycelium developed from the primary inoculum spreads rapidly 
in the succulent tissues of the seedlings, secreting enzymes which enable 
it to penetrate the cellulose walls, and toxines which kill the protoplasm. 
Study thin longisections (freehand or prepared) through diseased stems. 
OBSERVE : — 

6. The relation of the mycelium to the host-cells; inter- or intra- 
cellular ? Are haustoria produced ? 

7. The structure of the mycelium; densely granular, branched, 
non-septate, often knobbed and swollen. The form and structure may 
often be better made out in mounts from pure culture. 

Make and study mounts from cultures provided, observe: — 

8. That frequently there is a streaming of the cytoplasm in the 
long non-septate cells. 

DRAW, showing structure of the mycelium and its relation to the host- 
tissues. 

From this mycelium is sent forth from the diseased tissues short fruit- 
ing branches, — the conidiophores, on which are developed conidia just 
like those produced from the oospore. Make mounts from diseased 
stems or from pure cultures showing conidia. Study and observe: — 

9. Their peculiar form, apical papillae and coarsely granular 
contents. 

10. Their attachment to the conidiophore at their larger 
end. DRAW showing conidiophores with conidia. These conidia function 
as the inoculum for the secondary cycles. In P. de Baryanum it is held 
that mycelium spreading through the soil from seedling to seedling usually 
functions to initiate the secondary infections. This may also be true 
at times for P. Cactorum. 

Saprogenesis. The mycelium in the dead tissues continues to 
grow and produce conidia so long as the food supply is not exhausted. 
As the nutrients begin to fail there develops on the mycelivim within the 
tissues the sexual bodies, which eventually mature into oospores. Tease 
apart in a drop of water bits of diseased stems which contain forming 
oospores or use mounts from pure cultures provided, observe: — 

11. The large globose bodies, — the oogonia, and the smaller 
closely appressed antheridia. 

12. The manner in which they are borne on the mycelium and 
their positional relation to each other. 

13. Their thin, h^^alin walls and granular contents. 

14. The aggregation in some, of the denser protoplasm in the 
center, — the oosphere, and the surrounding layer of hyalin protoplasm, — 
the periplasm. 

15. The impregnation-tube penetrating from the antheridium, 
through the oogonium wall and periplasm to the oosphere. What is the 
function of this tube? 



30 

Make drawings showing the structure of the sexual bodies and fertili- 
zation of the oosphere. 

The fertilized oospores now go into a resting-period and will not germi- 
nate at once. Normally this dormant period extends through the winter. 
During the spring, changes in the oily protoplasm of the oospore take place 
which make its germination possible. With germination of these oospores 
(already studied) the primar^^ cycle is completed. 

Secondary Cycles are initiated repeatedly during the growing-season 
as has already been pointed out, either by conidia or by mycelium spread- 
ing through the soil. If viable conidia are available study conidial germina- 
tion on the slides provided; or Cornell Bui. 363, fig. 16-17; and read p. 97-99. 

Make drawings illustrating ways in which conidia (sporangia) germi- 
nate; also the structure and germination of the swarmspores. How do 
the germtubes gain entrance to the host- tissues ? Why can they not 
penetrate mature tissues? 

The further developments of the pathogene during the secondary 
cycles duplicate those of the primary. 

Pathological Histology. The pathologic effects of these pathogenes 
are of the simplest sort — a direct killing of the protoplasts — necrosis. 
Study sections through the advancing margin of the lesion, using fresh 
material, observe: — 

16. The plasmolyzed and disorganized cell-contents. 

17. The water-soaked appearance of the diseased tissues due to 
the diffusion of the cell-sap into the intercellular spaces, forcing out the 
air. 

DRAW to show these effects. 

Pathogenicity Studies. The rapid destructiveness of these pathogenes 
in a bed or flat of closely crowded seedlings is strikingly brought out by a 
simple inoculation experiment. 

Under the direction of the instructor each student will inoculate 
the seedlings provided, with pure cidtures of one or the other of these 
fungi. Keep under moist conditions and examine daily for first evidences 
of infection and successive symptoms. Make notes on conditions under 
which the inoculated plants were kept, length of incubation-period, 
effects of removing plants to dry conditions before all go down and other 
facts observed. 

REPORT 

1. Describe in detail and explain the functions of the structures 
developed by either of these pathogenes during its life-cycles. 

2. Prepare a complete report on the pathogenicity-studies 
made. 



LATE BLIGHT OF POTATOES 

This is the most destructive disease of the potato in the humid temperate 
regions of the world. It is especially severe in northeastern United States. 

vSYMPTOMS 

During damp, mugg^^ weather in August and September, circular 
areas in the fields are often to be obser\^ed in which all the tops of the pota- 
toes are dead or dying (photograph 1). (See also N. Y. (Geneva) Bui. 241, 
pi. VII, and 264, pi. II, III, VIII, XIV, XV.) 

On the leaves. Examine the material provided and observe : — 

1. The large spots or lesions with indefinite margins. Their 
position in the leaflets (photograph 2). 

2. On holding them to the light, the dark water-soaked or opaque 
appearance of the diseased areas. 

3. On the lower surface of the leaflet, a white frost -like mildew 
about the margins of the diseased spots. (See illustration specimen; photo- 
graph 2; or N. Y. (Geneva) Bui. 241, pi. XI.) In dry weather this mildew 
may be difficult to see. 

4. That the lower leaves are usually the first attacked. They 
die and shrivel on the stem. (See illustration specimen; or N. Y. (Geneva) 
Bui. 241, pi. X.) 

Compare these leaf-spot symptoms with those of the Altemaria blight 
as shown in the material and photographs provided. 

Show in DRAWINGS the difference in leaf -symptoms of these two diseases. 
Show both sides of the spots. 

On the tubers. The disease may also occur on tubers, causing a reddish 
brown rot. (Read Connecticut Ann. rept. 1904:364, second paragraph 
and see pi. XXXIII; also photographs 4a and 4 b.) 

Study the material provided and observe : — 

5. The sunken spots on the surface of the diseased tubers 
provided. To what due? 

6. The color and consistency of the diseased tissues shown on 
cutting the tuber open. How deep do the lesions extend? 

DRAW to show tuber-symptoms as observed in specimens and photo- 
graphs. 

This disease is also destructive to the tomato in some sections of the 
United States. If material is available study the symptoms on this host 
and show in drawings. (See California Bui. 175:9 and fig. 3, 4 and 5; 
and Virginia Bui. 192:7-10 and fig. 5, 6, 7 and 8.) 

ETIOLOGY 

One of the phycomycetous fungi, Phytophthora infestans (Montagne) de 
Bary, is the cause of this disease. The species of Phytophthora are all 
very virulent and destructive pathogenes. 

Life-history. So far as is certainly known this pathogene remains 
in continuous association with the living host throughout its life-cycles. 
The Primary Cycle originates from the activity of the mycelium in 
diseased tubers after they are planted in the spring. 

31 



32 

Pathogenesis. The fungus passes the winter as mycelitim in 
diseased tubers. Make thin tangential sections of the diseased tubers, 
stain with eosin and observe: — 

7. The mycehal threads between the cells. Their densely- 
granular contents. Note the absence of hatistoria. (Read Connecticut 
Ann. rept. 1904:364, last paragraph.) 

8. The general absence of septa in the mycelium; this fungus is a 
cenocyte. Toxines secreted from this mycelium kill the adjoining host- 
cells and their soluble contents are set free for the fungus. 

DRAW, or COPY a part of fig. 6 in California Bui. 175 to show character 
of the mycelium and its relation to the cells of the tuber. 

The mycelium in the diseased tuber grows out into one or more of the 
shoots resulting in a systemic invasion. The diseased shoot is weak 
and dwarfed. From such shoots conidia are produced in great abundance. 
Such plants become centers of dissemination or sources of inoculum for the 
primary infections. After these conidia are produced and dispersed the 
fungus in these systemically invaded shoots dies with the host. (Read in 
Jour. Agr. Research 5:99-102.) copy pi. VI, fig. 1. 

The primary infections occur on the leaves; are relatively few and serve 
to afford inoculum for the secondary cycles. With the death of the leaves 
the pathogene in them perishes and the primary cycle is not completed. 
The lesions and pathogene-structures are like "those resulting from the 
secondary infections in the leaves. 

The Secondary Cycles arise from the conidia produced on the primary 
lesions. These conidia are distributed by wind during rainy or foggy 
weather, to nearby healthy plants where they germinate. 

Pathogenesis. Make mounts of these conidia from material 
provided (freshly blighted leaves or cultures, if available) and observe : — 

9. The shape, contents and color of conidia. These conidia upon 
germination usuall}^ act as sporangia, giving rise to swarmspores. 

Study germinating spores, if available; N. Y. (Geneva) Bui. 141, pi. 
XII, and California Bui. 175, fig. 8; or photograph 5. make out: — 

10. The manner of gemiination ; number and form of swarmspores 
and their motility by flagella. 

DRAW to show points observ^ed or copy from plates or photographs 
studied. 

After swimming about for a short time in the drop of rain or dew on 
the host -leaf , the swarmspores come to rest, round up, invest themselves 
with a thin cellulose wall and germinate by a germtube. Study slides; 
figures in the bulletin; or photographs 5 and 6. observe: — 

11. The relative size of the germinating swarmspore and the 
conidium. The difference in form of the active and germinating swarm- 
spores. 

12. The two methods of entrance of the swamispore-germ- 
tube into the leaf, copy to show this. 

This germtube gives rise to mycelium within the leaf-tissues like that 
observed in the tuber. Haustoria-like branches are often sent from the 
intercellular mycelium into the leaf-cells. From this mycelium, branches 
are put forth on the underside of the leaf to form the conidiophores. 

Scrape some of these conidiophores from the leaf and mount in water. 
observe : — 



33 

13. Their form and structure. Distinguish the conidiophores 
from the large leaf-hairs. Note the peculiar swellings on the branches. 

14. The ovate conidia; their manner of formation and their 
attachment to the conidiophores. (See also photograph 5 or N. Y. (Geneva) 
Bui. 141, pi. XII.) 

If freshly diseased leaves are available, peel epidermis from lower 
surface of spot. Mount and study, making out: — 

15. How the conidiophores emerge from the tissues of the leaf. 
(See also N. Y. (Geneva) Bui. 241, pi. XII, fig. 2; Maine Bui. 169, fig. 15 
or California Bui. 175, fig. 7.) 

Make a drawing to show the leaf-surface, conidiophores emerging 
and conidia attached both at the apex and side of the branch. 

These secondary conidia in turn give rise to other secondary infections 
on healthy leaves or they fall to the ground during rains and produce 
swarmspores in the soil. These find their way to the growing tubers which 
they penetrate by a germtube. This gives rise therein to the myceliiim 
that is to carry the pathogene over to the next season and invade the shoots 
and develop conidia for the primary infections. Thus are the secondary 
or at least the late secondary cycles completed. 

Saprogenesis. As already indicated, a true saprogenic develop- 
ment of this pathogene in nature is not known. It will, however, grow as 
a saprophyte on certain culture-media. Here it will sometimes produce 
not only conidia but also resting-spores, called oospores. These have 
never certainly been obser\^ed in the tissues of the host. One might 
expect to find them in the dead tops killed by the pathogene or in the 
rotting tubers since other species of Phytophthora regularly fonn oospores 
which are long-lived and . serve to carry the fungus through the winter 
or drought -periods. Worthington G. Smith thought he had discovered 
them. (See Diseases of Field and Garden Crops p. 295-310.) No obser\^er 
since Smith has been able certainly to find them in nature. Jones of Ver- 
mont, found in his pure cultures, bodies which are strikingly like the 
oospores figured by Smith. (See Science 29:271.) Finally Clinton of 
Connecticut, succeeded in growing the oospores of P. infestans on oat-agar, 
and has even produced what he believes are hybrid oospores of P. infestans 
with P. PhaseoU and also with P. Cactorum. (See Connecticut Ann. rept. 
1909-1910:753-774, pi. XXXVIII-XL.) 

REPORT 

1. Discuss tuber-infection; where and how it occurs; two 
methods of prevention, explaining why each method is efficient. 

2. Give detailed directions for the control of the disease. 



DOWNY MILDEW OF CUCURBITS 

This is a well known and much dreaded disease of cucumbers and musk- 
melons, ranging from the tropics to the north temperate regions. It 
sometimes affects squashes and watermelons. 

SYMPTOMS 

The lesions of this disease are confined to the leaves. The oldest leaves 
are first affected. The fruit is dwarfed by the loss of food-supply. 

The general effect of the disease on the plant as a whole is quite striking. 
The affected leaves at the center of the hill, yellow and soon die and shrivel 
lilce leaves killed by the frost. (Study Connecticut Ann. rept. 1904, 
pi. XXX and read N. Y. (Geneva) Bui. 119:158.) 

On the leaf. Study the specimens provided, observe: — 

1. The location or distribution of the spots in the leaf -blade. 

2. The form, size, nature and color of the spots, both above and 
below. In the fresh condition the lower surface of the spot shows a pur- 
plish tinge due to the color of the conidia of the fungus. This color is lost 
in drying. 

3. Relation of the margin of the lesions to the leaf -veins. Hold 
to the light to see this. 

Make a drawing to show the character of the spots on upper and 
lower surfaces. 

4. That after the spots have coalesced the entire leaf becomes 
brown, shrivels and drops. 

Make a drawing of a leaf showing this advanced stage of the disease. 

ETIOLOGY 

This disease is caused by the phycomycetous fungus Peronoplasmopara 
cubensis (Berkeley and Curtis) Clinton. It is one of the "downy mildews." 

Life-history. Unlike most of the members of the order Peronosporales, 
P. cubensis appears to develop no sexual or resting-spores (oospores). 
It is believed that it depends entirely on conidia for reproduction, passing 
the winter on living hosts in the tropical islands of the West Indies or the 
extreme southern United States, spreading northward with the advancing 
summer weather. It ma}^ in some cases winter on greenhouse cucumbers. 
There are then strictly speaking no primary cycles. 

Secondary Cycles initiated in the south repeat themselves, spreading 
northward so that they reach the region of northern United States in late 
July and August. No saprogenic phase is certainly known for this patho- 
gene. 

Pathogenesis. The conidia produced in abundance on the lower 
surface of the lesions constitutes the inociiltim. Scrape some from the 
diseased leaves provided, mount and observe: — 

5. The large lemon -shaped conidia each with a papilla at the 
tip. The thin wall and the brown granular appearance of the contents. 
Make an enlarged drawing of a single conidium. 

6. That the conidia germinate by the formation of swarmspores. 
COPY the figures showing the different stages of germination from Connec- 
ticut Ann. rept. 1904, pi. XXXI. 

34 



35 

These swarmspores swim about in the water on the leaves for a short 
time, then round up, secrete a wall about themselves and send forth a germ- 
tube which penetrates the leaf and develops a mycelium within the tissues. 
The germtube may enter through a stoma or bore directly through the 
walls of the epidermal cells, copy the figures of swarmspore-germination 
and penetration from Connecticut Ann. rept. 1904, pi. XXXI. 

The myceliimi in the leaf is difficult to study, and as it is of the same 
character as that of the pathogene of grape mildew, the grape stem will be 
used instead. Cut a thin longitudinal section of the stem provided, 
stain slightly with eosin and observe : — 

7. The granular mj-celial threads between the cells. The my- 
celium is cenocytic. 

8. The button-like haustoria which branch from these threads 
and push through the cell-walls of the host. 

Make a drawing illustrating the mycelium with some of the host-cells 
and the haustoria pressing through the walls. 

The mycelium rapidly kills the tissues and as it does so it sends forth 
through the stomata on the under surface of the lesion long branched 
fruiting stalks, — the conidiophores. 

Scrape with a scalpel several spots on the under side of the leaf and 
place the substance on a slide in a drop of water. With the high-power, 
observe: — 

9. The long sparingly branched conidiophores with slightly 
swollen base, and the beak-shaped tips at the ends of the branches, to which 
are attached the conidia. 

Make an enlarged drawing of a conidiophore, showing conidia at- 
tached. 

These conidia are disseminated by the wind and initiate new secondary 
cycles. 

This pathogene sometimes attacks the wild cucumber, Echinocystis 
lohata (Michx.) T & G., but there is on this host another downy mildew 
pathogene, Plasniopara australis (Spegazzini) Swingle. To show the 
difference between these two pathogenes examine diseased leaves of 
E. lohata provided and observe:— 

10. The downy white spots on the lower side of the cucumber- 
leaf. 

Make a drawing of a leaf showing one of these spots. 

Place some of the fungus on a slide and with the high-power, observe : — 

11. The short conidiophores branched at right angles; the blunt 
tips with short sterigmata ; and the smaller almost globose conidia. 

]\Iake a drawing showing the points observed. 

report 
1. Discuss fully the control of the downy mildew of cucurbits. 



SCLEMOTIMA MOT 

This is a very common disease of a great variety of plants. It is especi- 
ally destructive in rainy seasons or in very moist situations. Due to the 
different symptoms exhibited in a great variety of hosts it has come to 
be known under several names as ; stem-rot in seedlings, soft rot in stored 
roots and bulbs, and "drop" in lettuce. 

SYMPTOMS 

The most ustial point of injury is the base of the stem, but the leaves 
of such plants as cabbage and lettuce, especially when packed together 
in heads are commonly affected. Fleshy roots, when stored together often 
suffer from this disease. Pods, fruits and twigs sometimes show lesions. 

On the stems. The name stem-rot is applied to the disease in those 
cases where the lesions are chiefly at the base of the stem. The symptoms 
are in general the same on the different hosts. Examine the diseased 
stems provided, observe: — 

1. A bleaching of the normal color of the tissues accompanied 
by a softening and sometimes a water-soaked appearance. If fresh 
material is available,, examine for these characters. 

2. The appearance, in a short time if the air is moist, of a densely 
matted pure white felt of mycelium on the surface of the lesion. Examine 
diseased specimens which have been kept for a time in a moist-chamber. 

3. The large black bodies, — sclerotia, on the surface of the lesion 
or more frequently within the pith of the stem. These sclerotia are at 
first soft cream-colored bodies embedded in the white m3^celial felt or in the 
pith. They soon turn black and become firm and tuber-like. 

Make sketches to show these distinguishing symptoms. 
On the heads of lettuce. "Drop" is frequently a serious disease of 
lettuce. Examine the diseased plants provided and observe : — 

4. The collapsed and wilting state of the plant. It has "drop- 
ped." SKETCH a diseased and healthy plant. 

5. The point of injury ; extent of the lesion. 

6. The soft slimy condition of the affected tissues. Note also 
odor, color, and consistency of the affected parts. How does this rot 
compare with the bacterial soft rot? 

7. The loose white felty masses of mycelitim, on and between the 
decomposed leaves. Larger masses of very compact aggregations of 
mycelium which are the beginnings of sclerotia. 

8. In somewhat older material the mature sclerotia which have 
become very hard with black, tough outer walls. 

■ On fleshy roots. As a storage-rot of carrots and turnips, this disease 
is often exceedingly destructive. Examine the diseased roots provided. 
OBSERVE : — 

9. The white felty mat of mycelium covering the surface of the 
lesion. 

10. The hard tuber-like sclerotia ; some mature black and hard, 
others just forming as white cottony masses on the mycelial felt. 

11. The soft decay of the tissues beneath the mycelial felt; 
odor, color, taste. 

36 



37 

12. The water-soaked zone in the tissue about the soft myceHum- 
coated center of the lesion. 

DRAW to show the external appearance of rotted roots. 
Cut the root lengthwise through the lesion, observe : — 

13. The depth to which the lesion extends. What tissues of the 
root are involved? 

DRAW to show internal characters of the rot in the roots. 

ETIOLOGY 

The disease is caused by the ascomycete, Sclerotinia Libertiana Fuckel 
(= Sclerotinia sclerotiorum (Libert) de Bary). 
Life-history. 
Primary Cycle. 

Pathogenesis. The source of inoculum for the primary cycle is 
the apothecium developed in the spring from the overwintered sclerotium. 
Examine the specimen provided, observe: — 

14. The large black sclerotium and the trumpet-shaped apothe- 
cium with stalk and concave disk. draw. From this apothecium are 
shot forth the ascospores which constitute the inoculum of the primary 
cycle. 

The character of this inoculum may be determined as follows : Gently 
break the seal and remove the cover from the glass preparation-dish 
in which are living apothecia. Watching sharply, observe : — 

15. The tiny cloud of spores that puff from the disc of the apothe- 
cium. This cloud is composed of ascospores which caught by the breeze 
are borne about and lodging in a suitable infection-court initiate the cycle. 
All of the ascospores of an apothecium are not discharged at one time. 

With your forceps carefully remove a bit from the edge of the apothe- 
cium and crush in a drop of water on a slide. Cover and study, observe : — 

16. The ascospores; their form, number in each ascus, and color. 
draw to show several ascospores much enlarged. 

Once within the infection-court where sufficient moisture is available, 
these germinate forming a germtube which soon branches to form mycelium 
and finally penetrates into the host-tissues. Examine the germinating 
ascospores provided, observe : — 

17. From what place on the spore the germtube arises; relative 
size of spore and germtube, branching, contents, draw. This germ- 
tube branches forming the mycelium which spreads through the tissues, 
causing the rot. 

Remove a bit of the rotten tissue and tease apart in a drop of water on 
a slide. Cover and examine for mycelium, observe: — 

18. Size, septation, branching, color, relation to host-cells, 
(inter- or intracellular?). Do haustoria occur? 

As this pathogene grows readily in culture, the mycelium may be 
advantageously studied in the agar-culture in the petri-dish provided. 
Carefully cut out a small square from the advancing margin of the colony, 
transfer to a slide in a drop of water, cover and with little or no crushing, 
study the mycelium, comparing this with that found in the tissues. 

draw to show the characters of the mycelium of 5. Libertiana. 

Sapro genesis. No conidia are probably ever produced by this 
fungus during its normal life-cycles; certainly not during pathogenesis. 



38 

Sometimes in very old artificial cultures or where the ascospores germinate 
in non-nutrient media, so-called microconidia are produced. (See text.) 
These have never been observed to germinate. 

When the mycelium has killed the tissues, it rapidly extracts and appro- 
priates the food-substance therein. As this food-supply begins to dwindle 
the mycelium forms gnarls of short intertwining hyphae at different points 
in or on the lesion. These rapidly enlarge, forming the mature black 
sclerotia. 

Make thin sections of one of the sclerotia and observe: — 

19. Its structure; (a) the black outer layer or rind made up of 
about two or three layers of cells whose walls are thick and brown to 
black; (b) the inner white medulla made up of densely interwoven thick- 
walled hyphae. These sclerotia are storage-organs and the food- sub stance 
stored is largely in the thickened walls of these medullary hyphae. 

DRAV/ to show structure of rind and medulla much enlarged . (See de Bary , 
Morph. and Biol. Fungi p. 30-32, fig. 14.) 

During a drought or cold weather all the mycelium, except that in these 
sclerotia, dies. The sclerotia pass the winter in the soil or old host- 
debris, in a dormant condition. In the spring they germinate; that is, 
from them develop the apothecia in which are produced the ascospores. 
(See de Bary, Morph. and Biol. Fungi p. 52-53, 218-219.) 

Make thin sections through the apotheciimi provided or use prepared 
slide. MAKE out: — 

20. Asci with discharging-pore at apex; ascospores, paraphyses, 
and hypothecium; also structure of stipe and excipulum. draw a 
portion of the section to show these sti~uctures in the proper relation to 
one another. 

Secondary Cycles. These may originate from mycelium which 
spreads through the soil from diseased to neighboring healthy plants. 
In this case the mycelium serves as inoculum. Aside from this the secon- 
dary cycles duplicate the primary. 

Pathological Histology. Mount a bit of the diseased tissue from just 
back of the advancing margin, determine: — 

21. The effects of the pathogene on the host-cells, draw to show 
this. 

This mycelium secretes a toxine which kills the host-cells. De Bary 
believed he had shown this to be oxalic acid. When the sclerotia are 
forming, large drops of liquid are extruded upon their surfaces. This 
also contains the toxine. 

If cultures showing these drops on the sclerotia are available, remove 
with the end of a needle-handle one of the drops to a clean slide ; quickly 
place a small drop of calcium nitrate solution beside it, place under the 
low-power and observe: — 

22. The formation of calcium oxylate crystals as one causes the 
two drops to come in contact, using a needle. Cover and study these 
crystals with the high-power. 

The mycelium also secretes a cytolytic enzyme which dissolves the 
middle-lamella from between the host-cells. 

Make thin sections of the young succulent portion of the host and mount 
in a large drop of the liquid from a bouillon-culture of 5. Libertiana. 
Mount similar sections in water. Cover in a petri-dish to prevent evapora- 
tion. After an hour or so, examine: — 



39 

23. Compare the two mounts as to changes in the tissues. 
RECORD the results in the form of notes or sketches. 

Pathogenicity studies. The pathogenicity of 5. Libertiana and the 
development of the symptoms of the disease may be easily studied as 
follows : — 

24. Inoculate at the base of stems of beans, sunflower or lettuce 
with an agar-block containing mycelium (from petri-dish cultures). 
Cover inocultmi with moist cotton and wrap with strip of tinfoil. 

25. Inoculate roots of carrot or turnip by placing inoctdum on 
the surface (injured or uninjured), cover with a bit of wet cotton and place 
the roots in a moist-chamber. 

26. If living ascospores are available, inoculate some- host-plants 
with a suspension of these in water. 

KEEP NOTES on method of inoculation, character of infection-courts, 
length of incubation-period and development of symptoms. 

Reisolate 5. Libertiana from the lesions appearing on inoculated plants. 

REPORT 

1. Drawings and report on inoculation-experiments. 

2. Detail the best method of control of this disease in one of the 
crops it affects, explaining how the life-history of 5. Libertiana Fckl. war- 
rants the procedure proposed. 



BOTM¥TIS BLIGHT OF PEONIES 

This is a common and very important disease of the peony. There 
appear to be in fact two Botrytis blights so similar in symptoms as to 
be distinguished only by a careful examination of the pathogenes involved. 

SYMPTOMS 
On young shoots. Study photographs and specimens provided. 

OBSERVE : — 

1. That when the plants first come up in the spring shoots 
from six to ten inches high, suddenly begin to wilt and fall to the ground. 
The entire shoot quickly withers and dries. 

2. At the base of these wilting shoots, the brown rotten lesion 
usually involving the entire circumference of the stem and extending 
some distance above the surface of the soil. The wilting noted above 
is due to this basal stem-rot. 

3. Covering the surface of some of these lesions the greyish 
brown felty covering of conidiophores of the pathogene, appressed or 
broken away from the dry specimens. Very evident in fresh specimens. 

4. On some of the lesions in their more advanced stages, small 
black bodies the size of a pin-head imbedded in the cortical tissues; most 
usually found at the base of the stem where it was covered by the soil. 
These are the sclerotia of the pathogene. 

SKETCH (full page size) to show the symptoms in blighted young shoots. 

As the season advances shoots continue to wilt from lesions at their 
base. However, the proportion of such cases decreases rapidly as the 
stem-tissues mature and harden. Toward the middle of the season the 
stems attacked at the base wilt but do not fall over. The development 
of the lesion is much slower and the wilting is often preceded by a yellowing 
or a purpling of the foliage. 

Study such specimens and photographs as are available showing the 
characters of the stem -rot in the older shoots. 

On the buds. The bud-rot is the next symptom of the disease to develop 
after the stem-rot of the yoimg shoots. The symptoms exhibited by 
affected buds are of two general types; the "bud-blast" of the small 
immature buds when they are from the size of a pea to half an inch in 
diameter and the "bud-rot" which is the name applied to the disease 
as it manifests itself in affected buds after they are nearly or quite mature, 
just before blooming. 

Bud-blast. Examine the material available, observe: — 

5. Black or brown undeveloped small buds in the cluster. 
It is usually the first or leader-bud of the shoot that is affected. Note 
that some of the secondary buds are larger and healthy. They grow 
rapidly after the death of the main-bud. 

6. The brown felt of conidiophores with grey conidia. This is 
commonly wanting or difficult to detect with the naked eye especially 
in dry specimens. 

SKETCH to show bud-blast. 
Bud-rot. Examine one of the large buds provided, observe: — 

7. The general discoloration of the floral parts. The bud is 
killed. It fails to open. 

40 



41 

8. The felt of conidiophores, usually most noticeable about the 
base of the bud. 

9. That the lesion often extends down the stem for some dis- 
tance below the bud. (Study illustration specimens and autochromes.) 

SKETCH to show the external characters of bud-rot. 
Some of the dry rotted buds have been moistened to put them in a more 
characteristic condition. Carefully dissect one of these and observe: — 

10. That the innermost parts of the bud are involved, the petals 
being very soft and decayed. 

11. That the petals do not separate readily from one another 
being held together by the mycelium of the pathogene. By carefully 
tearing apart two adjoining petals the stretched and breaking hyphae may 
sometimes be detected. 

12. Mats or wefts of mycelium which may sometimes be ob- 
served in the cavities of the bud. 

The bud-rot phase of this disease is probably its most serious feature. 
Why? 

On the blossoms. Blossom-rot. Sometimes the half-open buds are 
attacked. They turn brown and collapse and become a rotted mass. 
This occurs only during an epiphytotic of the disease in very wet weather. 

Examine the dried specimens provided. They give only a partial idea 
of this condition as it appears in the field. Compare with healthy blos- 
soms as shown in the autochrome. sketch. 

On the upper stem. It has already been seen that the stem at the 
base of the bud may and usually does become involved. Examine speci- 
mens that show upper stem-lesions, observe: — 

13. The brown discoloration and shrunken character of the dis- 
eased stem. 

14. In many cases, very characteristic alternating light and dark 
bands or zonations especially marked on stems just below rotted buds. 
(See autochrome.) 

15. On some of the lesions, conidiophore-felts. These are 
usually wanting on stem-lesions. 

16. Occasionally stem-lesions showing the small sclerotia of the 
pathogene as small black bodies bursting through the epidermis. (Examine 
illustration specimens or photographs showing these.) 

17. That sometimes the pathogene spreads from blasted small 
secondary buds- or from blighted leaves into the stem, forming canker- 
like lesions. (Examine illustration specimens, photographs and auto- 
chromes.) 

SKETCH to show different types of upper stem-lesions. 

On the leaves. Leaf -lesions while they may occasionally appear early 
in the season become characteristic and abundant usually at the time of, 
or just after the blossoming-period. Examine specimens and photographs 
provided, observe : — 

18. The location of the majority of the lesions, i.e. the part of 
the blade involved. 

19. The average size and color characters of the lesions. Note 
evidences of zonation in some of them similar to those on the stem. 

20. Conidiophores. Where most abundant? That they are 
more scattered, i.e. less felty than on shoots and buds. 

sketch to show both surfaces of leaf -lesions. 



42 

On the root. It has recently been found that the crowns and roots 
may also become involved, resulting in the long-known "brown rot" of the 
roots. If specimens are available study, noting: — 

21. The firm more or less dry brown decay of the tissue. The 
entire diameter of the root is involved. Compare with specimens of the 
so-called "black rot" of the roots. 

ETIOLOGY 

There are two pathogenes which cause Botryris blights of peonies. 
The small-sclerotial species is Botrytis paeoniae Oudemans. The large- 
sclerotial species is Botrytis (species?). The Botrytis paeoniae Oudemans 
appears to be the more common and destructive and is the one here con- 
sidered. 

Life-history. This fungus exhibits in its life-history both primary 
and secondary cycles with characteristic pathogenic and saprogenic 
phases. 

The Primary Cycles are initiated and completed, chiefly at least, 
on the young shoots which become diseased shortly after they come up 
in the spring. 

Pathogenesis. So far as is known conidia alone constitute the 
primary inociiltim. These are produced in early spring from overwintered 
mycelium in the old diseased stubble or the stems on the ground. Pos- 
sibly also from germinating sclerotia in the old stems. 

Diseased stems have been brought in from the field and placed in moist- 
chambers. Examine the material and observe ; — 

22. The numerous tufts of brown conidiophores covered with 
grey conidia, — the primary inoculum. 

SKETCH to show thcsc conidial clusters on the stems. 
Mount some of the conidia and observe : — 

23. Their form, size and color. These conidia are carried by the 
wind and insects, chiefly ants, to the young shoots that are coming up 
nearby. Here they lodge and if conditions of moisture and temperature are 
favorable they germinate and penetrate the tender young tissues. Study 
the germinating conidia on the slides provided, draw. 

After penetration by the germ tube, infection occurs, the first evidence 
of injury showing in two or three days. The mycelium ramifies the tender 
tissues in all directions killing them presim:iably by toxines which it se- 
cretes. 

Make thin sections of diseased stems provided and study: — 

24. The morphology of the mycelium and its relation to the host- 
cells. Is it inter- or intracellular? draw. 

From the mycelium, conidiophores are soon thrust forth to the exterior 
and conidia quickly and abundantly produced. Make mounts of conidio- 
phores from diseased stems, observe: — 

25. Their form, structure and the way in which conidia are borne. 
draw. 

Sapro genesis. After the shoot dies the mycelium begins to form 
sclerotia at the surface of the lesion, especially below the surface of the 
soil, where moisture conditions are more favorable. 

Examine the sclerotia growing in pure cultures or on diseased shoots. 
compare : — 



43 

26. With the sclerotia of Sclerotinia Libertiana as to size, color 
and relation to substratum. Make comparative sketches. 

Crush or section one of the minute sclerotia otB. paeoniae and observe : — 

27. Its structure; the outer black rind-cells and the central white 
medulla, draw : 

The diseased shoots shrivel and fall to the ground, harboring the 
sclerotia until next season. The mycelium frequently spreads down from 
the diseased shoots into the crowns and roots causing root-rot. 

It appears probable that mycelitmi as such and not in the form of 
sclerotia, hibernates in the old tissues and even spreads through them as a 
saprophyte giving rise in the spring to conidia for the primary infections. 
Examine the old stems in the moist-chamber. They have just recently 
been brought in from the field, observe: — 

28. The numerous tufts of brown conidiophores covered with 
the grey conidia. From what do the conidiophore-clusters arise? Dis- 
sect one of them carefully away from the stem. Do they ever appear to 
arise from a sclerotium? 

Secondary Cycles. The conidia produced on the diseased young 
shoots are carried chiefly by ants to the developing buds where in the 
sugary exudate they quickly germinate, penetrate and cause the bud- 
blast. 

Other secondary cycles are exhibited later in the bud-rot, blossom-rot, 
upper stem-rot, and leaf -blight. The, fungous structures produced; 
their relation to, and effects upon the tissues are the same as those of the 
primary cycles. Sclerotia are rarely produced, however, in any of the host- 
organs other than the stem. The mycelium of the secondary cycles in 
old leaves and especially stems gives rise in the moist days of spring to 
great numbers of conidia which may also initiate primary cycles. 

report 
1. Write a letter to a peony -grower in reply to the query "How 
shall I proceed to control the Botrytis blight in my peonies?" It is to be 
assumed that the correspondent knows the various symptoms of the 
disease and that it is caused by a fungus. 



BMOWN EOT OF §TONE=FMUITS 

This disease is especially destructive on plums, peaches and sweet 
cherries. Apples, pears and quinces in America are sometimes affected. 
In the case of certain varieties of apples, affected fruits become jet 
black instead of brown. (See illustration specimen.) The brown color 
of the affected tissues of plums and peaches has given to the disease 
the name of brown rot. 

SYMPTOMS 

The disease is usually confined to the fruit, although it may appear 
also on other parts of the host, — blossoms, leaves, twigs and limbs. 

On the blossoms. The blossom-blight form of the disease is the 
first symptom to appear if the spring weather is at all favorable. Examine 
the specimens provided and observe: — 

1. Effect of the disease on the blossoms; similar to the effect 
of the fire-blight on apple blossoms. 

2. The grayish brown spore-tufts of the fungus on the petioles 
and young fruits. 

DRAW to show blossom-blight symptoms. 

On the fruit. Diseased fruits of plum, peach, cherry or apple have 
been provided. {Do not remove preserved specimens from bottles.) 
observe : — 

3. The brown discoloration of the diseased area. The margin 
of the spot, distinct or indefinite? 

4. That at first the diseased tissues do not collapse, the diseased 
portions retaining their original plimip character. Compare this with a 
fruit in an advanced stage of decay. To what is the difference due ? 

DRAW to show fruits in early and advanced stages of the rot. 
With the finger test: — 

5. The relative firmness of the diseased and healthy portions 
of the fruit; strength and character of the epidermis. 

Cut into the fruit and note: — 

6. The effect of the disease on the tissues as to discoloration, 
firmness, taste and odor. 

The fruit is often attacked and killed before it matures and for this 
reason clings to the tree, instead of falling as is the case with ripe or pre- 
maturely ripened fruit. Such diseased fruits dry and wrinkle forming the 
so-called mummies of brown rot. In the dry material provided observe : — 

7. The dry and shriveled condition of the fruit. 

8. Often a cluster of the mtimmies clinging together, boimd to 
each other by threads of mycelium where they touch. 

9. The tufts of the fungus crowded or scattered over the surface; 
color? 

sketch a mummied plum. 

On the twigs. In some seasons the twig-blight form of this disease 
is severe, especially on plimis and peaches. Examine the specimens 
provided and observe : — 

10. The dead and shriveled brown leaves of the upper part of the 
twig, in appearance not unlike the effect of fire blight on apple or pear 
twigs. 

44 



45 

11. The old blossom imbedded in a drop of giim at the point 
where the twig was girdled ; or — 

12. The old mummied fruit or fruit-pedicle where the twig was 
girdled. 

DRAW to show the twig-blight form of the brown rot. 

On the limbs. Brown rot cankers are very common on large and small 
limbs of peach trees in some peach-growing sections. They originate 
usually from the blighted twigs, blossom or fruit-spurs. Examine the 
cankered limbs provided and observe: — 

13. The rough open cankers with evidences of repeated callus- 
formation. What does this indicate as to the age of the canker? 

14. Evidences of the diseased twig or spur about which the 
canker developed. 

15. Cankers breaking out above or below the original lesion, — 
evidences of metastasis. 

DRAW to show the character of brown rot cankers. 

ETIOLOGY 

The pathogene causing the brown rot of stone-fruits is Sclerotinia 
cinerea (Bonorden) Woronin. A closely related species, Sclerotinia 
fructigena (Persoon) Schroeter, is the common cause of brown rot of 
apples, pears and similar fruits in Europe. These fungi belong to 
that group of the ascomycetous fungi known as the Pezizales which 
are characterized by having an open cup-shaped apotheciuin in which 
the asci are bome. In the conidial stage the pathogene has been com- 
monly known as Monilia cinerea Bonorden. 

Life-history. This fungus has a more complicated life-history than 
most pathogenes causing diseases of the more common crops. This is due 
to the fact that its conidia may live over winter and initiate primary 
cycles. 

The Primary Cycle may be initiated from more than one source 
and by two sorts of inocula, as brought out in (a) and (b) below. 

Pathogenesis, (a) The mummies which fall to the ground give 
rise in the spring to apothecia which discharge myriads of ascospores 
about blossoming-time. The apothecia are very short lived, appearing 
over a period of not more than two or three weeks. Specimens have been 
provided, observe : — 

16. The goblet-shaped fruit-body, — the apothecium with a long 
slender stipe. Upon what does the length of the stipe depend? 

17. The size and color of the apothecium. The upper surface 
(bearing asci) is when fully expanded, usually flat or even convex with 
recurving margins. They retain almost their natural color in alcohol. 

IS. The attachment of the stipe to the mummy. To which 
side, upper or lower, is it attached? Why? 

Make a diagrammatic drawing showing the anatomy of the apothe- 
cium, its relation to the mummy and to the surface of the soil. 

Crush a bit of one of the apothecia especially provided for this purpose 
and observe : — 

19. The minute ovate ascospores, — the inoculum. Some of them 
are still in the asci in which they are formed, draw. They germinate by 
a simple germtube. 



46 

(b) The mummies which ding to the trees through the winter or 
the cankers on the hmbs harbor the pathogene as living myceHtmi or as 
thick-walled conidia. The warm spring rains cause the formation of 
multitudes of thin-walled conidia which it is believed may also function 
as primary inoculum. Mummies have been brought into the laboratory 
and placed for several days in a moist-chamber. 

Examine one of the mummies and note : — 

20. The grayish brown powdery spore-tufts. Make an en- 
larged DRAWING of a part of a fruit showing these cushions as they appear 
under the hand-lens. 

With the point of a scalpel remove a bit of the spore-mass to a slide, 
cover and examine, observe: — 

21. The rather large hyaline globose or oval bodies scattered 
through the mount,— the conidia. Variations in form, size, and thickness 
of the walls. 

22. The granular, sometimes alveolate character of the proto- 
plasm of the spore. (See also under the oil-immersion demonstration 
microscope.) 

Make an enlarged drawing of a conidium to show the structure. 
Slides showing germinating conidia have been prepared. Examine and 
OBSERVE : — 

23. The long septate germtubes. Simple or branched? 
How many from each conidium? draw. 

The ascospores or conidia are carried to the opening blossoms, where 
if the weather be rainy they quickly germinate and the germtubes penetrate 
the calyx or exposed corolla. The blossom-blight form of the disease is 
thus initiated. 

Mount one of the diseased petals provided. Stain with eosin and 
STUDY : — 

24. The mycelium in the tissue; its form, size, septation and 
branching. Does it go through or between the cells? draw. 

The m^^celium ramifies throughout the blossom and on protruding 
conidiophores forms masses of grayish brown conidia, — the inoculum for 
secondary cycles. From the blighted blossom the mycelium follows down 
through the fruit-spur to the twig or branch kilhng the bark and causing 
twig-blight or young cankers. In the cankers on the limbs the fungus 
may reniain alive throughout the winter and provide, as indicated above, 
conidia] inoculum for the primary cycles the next year. 

The primary cycle does not include a saprogenic phase unless developed 
on fruits, which is probably rare. 

The Secondary Cycles are in all probability initiated from conidia 
developed on the blighted blossoms or from other secondary infections 
on fruits and twigs. The blighted twigs, blossoms and green fruit con- 
tinue to harbor and multiply the parasite during the early part of the 
season until the maturing fruit offers opportunity for its very general 
spread and destructive work. 

Pathogenesis. Diseased specimens of ri]5e fruits showing the 
pathogene are provided. With forceps, remove a bit of tissue from a badly 
rotted fruit. Tease apart in a drop of water on a slide, cover and study 
under the microscope, observe: — • 

25. The form, structure, contents, septation branching, size 
and color of the mycelium. Note variation in diameter of the mycelial 
threads. (Staining with eosin or m.eth^dene blue will bring out the mvcel- 



47 

ium.) Are the cells penetrated ? If available, study the mycelium growing 
in agar. 

DRAW to show the mycelium in the fruit and its relation to the cells. 

This mycelium soon amasses at certain points beneath the skin of the 
fruit and bursting the skin pushes forth a cushion of short branched 
conidiophores. Make thin sections through these cushions, — sporodochia, 
and the skin of the fruit (or use prepared slides), study and observe: — 

26. The cluster of conidiophores protruding through the rup- 
tured epidermis. The cuticle of the fruit pushed back on either side._ 

27. The densely packed mass of mycelial cells from which the 
conidiophores arise, — the stroma. Note the pseudoparenchymatous struc- 
ture. 

28. The conidiophores; their form and mode of branching, 
length and diameter, color, contents and septation. (Use material from 
the petri-dish culture.) 

29. The conidia; their mode of formation, — moniliform. It was 
from this character that the conidial stage first received the name Monilia. 

Make a careful drawing of the section through the sporodochium 
to show its structure and relation to the host. 

From the diseased fruits, especially if attacked when green, the my- 
celium may penetrate through the cortex of the fruit -pedicle to the bark 
of the twig and cause twig-blight. Then passing on down to the larger 
limbs, it spreads into the bark and initiates the large limb-cankers. 

Saprogenesis. Many of the rotted or mummified fruits fall to the 
ground in the autumn or cling to the tree. Those on the tree may, as 
already seen, harbor the parasite throughout the winter. Those which 
fall to the ground become more or less buried in the soil and in them just 
beneath the cuticle in the epidermal regions is formed a crust-like black 
stroma. Make sections (or use those provided) through the skin of one of 
these fallen black mummies. Study and observe : — 

30. The black thick-walled cells of the stroma. Their relation 
to the old dead host-tissue, draw. 

From this stroma there arise in the spring the apothecia, the external 
features of which have already been studied. Examine a thin section 
through the apotheciimi and observe : — 

31. The densely packed hyphae making up the greater portion 
of the section, — the excipulum. This is made up of much septate inter- 
woven threads of the mycelium. It forms the bulk of the apothecitmi 
and serves as a protection and foundation for the layer of asci that Hnes 
the cup, — the hymenium. 

32. The make-up of the hymenium; the long slender tube-hke 
asci containing the ascospores. How many ascospores in each ascus? 
Their form and structure; the thread-like paraphyses standing up between 
the asci, septate or not? 

The paraphyses at first form the hymenium but as the apothecium 
expands the asci are formed and push up between the paraphyses and 
become a part of the hymenium. Make out the pore at the apex of the 
ascus. Confirm your observations with the prepared sHdes. 

Make drawings to illustrate fully the structure of the apothecium. 

REPORT 

1. Prepare a graphic illustration (cartoon) showing the life-his- 
tory of Sclerotinia cinerea (Bon.) Wor. 



BEAN ANTHEACNOSE 

This is one of the most serio-us diseases with which the vegetable- 
gardener has to contend. It occurs wherever beans are grown and. under 
conditions favorable for the fungus, will ruin an entire crop. 

SYMPTOMS 
Lesions are found on all parts of the plant above the root. 
On the seedlings provided, observe : — 

1. The elongated, blackened lesions on the stems, with the 
centers shrunken and in some cases the host-tissue cracked open. 

2. The lesions on the cotyledons which resemble very much 
those on the pods. Where do the lesions first appear, on the stem or on the 
cotyledon? Why? 

Make a drawing of a diseased seedling. 
On the leaves provided, observe: — 

3. The browned or blackened lesion on the under side. The 
areas between the veins are seldom injured. On the young growing leaves 
the veins are often killed while the tissues between still continue to grow, 
so that the leaf becomes distorted. Do the lesions show on the upper side 
of the leaf? draw. 

On the pods provided, observe : — 

4. The distinct regularly outlined, much depressed spots on the 
pod; their shape and size. 

5. The margin of each, bordered with a red zone, and that 
where two spots are near together, the red zones coalesce. 

6. The blackened center and, occasionally in the larger lesions, 
the host-tissue so shrunken that it is cracked open. Are there any spots 
lacking the black centers? 

Make drawings of several lesions on a pod to show the above points. 
On the seeds provided, observe : — 

7. The browned or blackened areas. Do they resemble in any 
way those on the pods? Are they depressed? Is the seed-coat ruptured 
at any point in the lesion ? draw. 

Compare the symptoms of this disease with those of bean blight, shown 
in the illustration materials. 

ETIOLOGY 

The pathogene causing bean anthracnose is Colletotrichum Linde- 
muthianum (Saccardo and Magnus) Briosi and Cavara. It is one of the 
Melanconiales, a group of the Fungi Imperfecti. Its perfect form is not 
known but is doubtless an ascomycete of the genus GlomereUa. 

Life-history. 

The Primary Cycle is initiated by conidia from the lesions on the 
cotyledons. When diseased beans are planted they contain mycelium 
of the pathogene, which at the germination of the host also begins to grow 
and is ready to produce spores when the cotyledons push above the surface 
of the soil. 

Pathogenesis. Scrape some material from a lesion on the cotyledon 
of a seedling provided. Mount and examine, observe : — 

48 



49 

8. The numerous small hyaline conidia mixed with the debris 
of diseased tissues. Their shape, size and color, draw. These constitute 
the primary inoculum. They are washed or splashed by rain to the 
stalks and first leaves of the seedling which thus inoculated show within a 
few days the lesions of the primary infection. The cotyledons soon 
shrivel and fall carrying with them the source of primary inoculimi. If 
dry weather prevails from the time the beans come up until the cotyledons 
fall, few or no primary infections occur and a clean crop is assured. 

The conidia, washed from the cotyledon-lesions, germinate on the stems 
or leaves by a germ tube. This penetrates directly into the tissues or 
forms a resting-cell at its tip, — an appressorium, from which later a germ- 
tube penetrates the cell on which it rests. Study germinating conidia 
provided and observe: — 

9. The number and positional origin of the germtubes. Note 
that the identity of the conidium is nearly or quite lost in the developing 
mycelium. 

10. The darker thick-walled appressoria at the ends of some of 
the germtubes. What is their function? 

DR.\w germinating conidia. 

There are soon developed in the primary lesions on the seedling-leaves 
and stems, minute fruit-bodies. Examine the lesions and observe: — 

11. The number and location of these fruit-bodies, — the acervuli. 
Color? Some of them may show (if fresh specimens) the pink mass of 
conidia oozing out on the surface of the lesion, draw to show the appear- 
ance of these acervuli in the lesion as seen under the hand-lens. 

Mount and examine some of the conidia from a primary lesion to see 
that they are the same as those from the cotyledons. Conidia from the 
primary lesions are produced for a considerable period and initiate the 
secondary cycles. 

Saprogenesis. The mycelium in the old primary lesions may con- 
tinue to live and spread saprophytically into the dead tissues of the leaves 
and stems, on which in the spring it may produce conidia. These conidia 
probably seldom function in initiating primary infections. The sexual 
stage may develop from the saprophytic mycelium in old stems and 
leaves but is not certainly known for this pathogene. 

Secondary Cycles develop on the later leaves and on the pods. The 
same phenomena of inoculation and infection are exhibited as in the 
primary cycle. 

Pathogenesis. The lesions on pods afford the best material for a 
study of the structure of the fruit-bodies of the pathogene. 

Study cross-sections through a lesion, observe: — 

12. The saucer-shaped acervuli, at the surface of the lesion; 
their relation to the host-tissues, especially the epidermis. 

13. The thin mat of myceliimi forming the base of the acen^ulus, 
from which arise the conidiophores. 

14. The size and shape of the spores; the manner in which they 
are attached to the conidiophores. What are the bubble-like bodies 
found embedded in the granular contents of each? Do the spores have 
septa ? 

15. The dark tapering septate threads which extend above 
the conidiophores and have somewhat the appearance of spines. These 



50 

are the setae, which distinguish the genus Colletotrichum from Gloeo- 
sporium, another anthracnose-producing group of organisms. 

Make a diagrammatic drawing of a lesion showing the relation of the 
acervuli to the host-tissue. Make an enlarged drawing of a portion of an 
acervulus, showing its structure. 

If fresh pods showing the oozing spores are not available examine the 
ctiltures provided and observe: — 

16. The pink masses of spores embedded in a gelatinous matrix. 
It is in this manner that they ooze from the acervulus and are later washed 
or splashed by the rain to initiate new infections. They are formed so fast 
by successive constrictions of the conidiophores that many millions may be 
formed on one badly diseased pod. 

If conditions are favorable, the mycelium in the lesions on the pods 
grows through the tissue and into the seed below it. It is in this manner 
that the bean seed becomes infected with the fungus-hyphae which pro- 
duce spores on the growing cotyledons the following season. 

Cut through a bean-pod directly over a lesion and observe: — 

17. The depth of the diseased area and the relation which the 
pod-lesion bears to the discolored area on the seed. Make a drawing 
showing this relation. 

In this manner the pathogene usually completes the cycle without 
going through a saprogenic period of existence. 

Sapro gene sis. The mycelium in the pods and stalks develops 
saprophytically after they are harvested or fall to the ground. Here are 
produce conidia whenever sufficient moisture and favorable temperature 
allows. If such debris is scattered wdth manure on land planted to beans, 
conidia produced thereon might cause primary infections in the spring. 
Observation indicates that this rarely occurs and that diseased seed is 
the usual source of the primary inoculum. 

Pathological Histology. Study cross-sections through lesions on 
cotyledons, stems or pods. (If freehand, stain with methyl blue or 
eosin. Wash thoroughly.) observe: — 

18. The broken-down mass of cells causing a deep depression 
in the tissues, leaving a blackened layer of disintegrating cell-walls at the 
base of the depression. 

19. The numerous mycelial threads which are both inter- and 
intracellular, sometimes almost completel}^ filling the cell; the individual 
hyphae which are septate and much branched, and the granular contents 
with ntunerous bubble-like bodies scattered throughout the threads. 
Are these bodies vacuoles or nuclei ? 

20. The effect on the cell-contents, i.e. protoplast, starch or 
chloroplasts. 

Make a drawing showing points brought out above. 

report 

1 . Outline a method of eradication by which clean crops of a sus- 
ceptible variety of beans may be eventually obtained, starting with diseased 
seed ; (a) for a northern grower ; (b) for a grower in the Gulf states or the 
southwest. 



ANTHMACNOSE OF SYCAMORES AND OAKS 

Th anethracnose of sycamores and oaks is a serious disease, especially 
in the nort;heastem part of the United States where it is frequently epiphy- 
totic. It is enphytotic on most of the species of the genus Platanus. 
In Europe as well as the United States the sycamores are more commonly 
and destructively attacked than are the oaks. However, in certain regions 
some species of oak suffer severely. 

SYMPTOMS 

The leaves, twigs and small branches are affected. In addition to the 
characteristic lesions produced on these parts of the tree, there are certain 
general symptoms by which a diseased individual may be detected at a 
distance. 

General S5miptoms. In the photographs provided, observe: — 

1. The dead, leafless twigs. 

2. The irregularity of the direction taken by smaller branches. 
These twisted and deformed branches are characteristic of this disease. 

3. The excessive twigging on the smaller limbs, producing clumps 
of branches, — witches'-brooms. Note that many of the branches of 
the broom are dead. Make a drawing of the broom-like growth of the 
branches. 

In the case of trees badly affected, the foliage becomes thin due to the 
falling of many leaves, and those remaining appear scorched owing to the 
large dead areas in them. In the early spring when the leaves are develop- 
ing, all the leaves on a twig may wither and die due to the death of the 
twigs. If this type of injury is general it may be confused with frost- 
injury. 

On the leaves of sycamore. In the material provided, note : — • 

4. The large brown areas in the leaves, — the lesions. 

5. That usually the lesion is located on one of the larger veins 
of the leaf. 

The lesions, when first noticeable, are narrow dead areas paralleling 
the veins on both sides. This dead area then rapidly becomes larger as 
the tissues of the vein are killed and the portion of the lamina dependent 
on the vein for water dies and turns brown. Why do most of the lesions 
extend to the margin of the leaf? 

6. The lesions on the petioles and the pimple-like fruit- 
bodies of the pathogene. 

DRAW sycamore leaves showing these symptoms. 

On the leaves of oak. In the material provided, observe: — 

7. That in some cases the lesions are similar to those on sycamore 
leaves in that the dead areas are located along the veins; but that more 
commonly numerous small lesions occur without any relation to the posi- 
tion of the veins. How can the difference in size between the lesions 
not located on the veins and those located on the main veins be accounted 
for? Make a drawing of an oak leaf showing both types of lesions. 

8. On the under surface of the lesions, minute brown spots, — the 
fruiting structures of the pathogene. draw as seen with the hand-lens 
or low-power of the microscope. 

51 



52 

On twigs. Twigs from oak or sycamore, collected in the early spring, 
are provided, observe : — 

9. That the twigs are (or were) still alive and the buds formed 
the previous autumn have reached maturity. 

10. The fruiting stage of the pathogene formed under the cork- 
layer, raising it up in pimple-like pustules. Later the cork-layer is rup- 
tured and the fruit-bodies are exposed. 

11. The terminal portion of the twig is partially or entirely 
invaded by the pathogene, as indicated by the distribution of the fruit- 
bodies noted above. 

Twigs thus affected may put out leaves in the spring which, however, 
usually wither and die before they reach full size. This is due to the rapid 
advance of the pathogene in the twigs, killing them. 

On the branches of sycamores. In the affected branches of sycamore 
NOTE : — 

12. The small diseased areas around the bases of the twigs, — 
cankers. 

13. Larger cankers with scars of old twigs at the center, showing 
the origin of the canker. 

14. The cortex of the bark raised up and broken through by 
the fruit-bodies of the pathogene. In some cases these fruit-bodies 
are gone and only a cavity is left in the bark. 

15. Older cankers in which the dead bark has fallen away and 
the wood is left bare. 

16. That callusing takes place rapidly in some cases and the 
open wound is covered. 

17. That swellings of irregular shapes are often formed at points 
where witches'-brooms originate. 

ETIOLOGY 

The anthracnose of sycamores and oaks is caused by the ascomycetous 
fungus, Gnomonia veneta (Saccardo and Spegazzini) Klebahn. • Previous 
to the discovery of the ascigerous stage of the pathogene, it was known 
by several names because of the variable types of asexual fruit-bodies 
which the fungus forms. The commonest of the names for the imperfect 
forms is Gloeosporium nervisequum (Fuckel) Saccardo, this being the name 
for the acervulus-stage which occurs on the lesions on the leaf-blades 
and petioles. On dead leaves, a pycnidial stage is formed, — the 
Sporonema stage and on the twigs, a pseudopycnidial form, — the Myxospor- 
ium stage. 

Life-history. The polymorphic character of the asexual fruit-bodies 
found on the different parts of the hosts which are attacked, makes the 
life-history of this pathogene complicated. Their identity was established 
by Klebahn in 1905. Klebahn also connected the polymorphic conidial 
stage with the ascosporic stage which is developed on overwintered leaves 
on the ground. 

The Primary Cycle may be initiated by several sorts of inocula. No 
definite investigations have been made to determine the relative effective- 
ness of these in nature. The possible primary inocula may be, ascospores 
from overwintered leaves, conidia from the Myxosporium stage on twigs 
and cankers, and conidia from the Sporonema stage on overwintered 



53 

leaves. It seems most probable that the primary inocula in the spring 
are the conidia from the twigs and cankers or the ascospores in the over- 
wintered leaves. 

Pathogenesis. The fungus passes the winter in the living twigs. 
In autumn, fruit-bodies begin their development on the affected twigs 
and reach maturity early in the spring. In prepared sections of develop- 
ing fruit-bodies on oak twigs, note : — 

18. That, while in the lower half of the fruit-body an opening 
has formed, the upper half is made up of mycehum and is still covered 
by the cork-layer of the bark. 

Before early spring the mycelium in the upper part of the fruit-body 
disappears and the cork-layer is ruptured so that the conidia may escape. 
Examine with the hand-lens the twigs with the mature Myxosporium 
stage. Make freehand sections at right angles to the twig. Stain with 
eosin, mount and observe: — 

19. That the top of the fruit-body is open. Has it a definite 
ostiolum ? 

20. The abundance of conidia. 

21. The size, shape and color of the conidia. 

These conidia are produced in great abundance and ooze out of the fruit- 
bodies in creamy drops or strings. In the presence of abundant moisture 
they may be washed or splashed to the young unfolding leaves. It seems 
that the conidia thus disseminated in early spring from the fruit -bodies in 
the twigs are the most efficient of the various possible primary inocida. The 
other types of primary inocula undoubtedly play some role and must be 
considered. In the overwintered leaves, specimens of which are provided, 
NOTE : — 

22. The small pimple-like bodies on the surface. 

Remove one of these fruit-bodies to a slide, mount and crush, observe : — 

23. The abundance of conidia coming from the pustules. 

24. The remains of the walls which enclosed the spores. 

25. The size, shape and color of the conidia. Compare with the 
conidia obtained from fruit-bodies on the twigs. 

Make drawings of this stage. 

The pycnidia-like fruit-bodies thus formed on overwintering leaves 
come about by growth of the vegetative hyphae around the margin of the 
acervulus, arching-up over the conidia until they are enclosed. This is 
the Sporonema stage and is formed on leaf -petioles as well as on the blade. 
Study the Sporonema stage on old petiole-lesions if material is available. 

DRAW. 

The conidia in the Sporonema pustules on dead leaves remain viable 
through the winter and may cause primary infection in the spring if they 
reach susceptible parts of the hosts. 

In the same overwintered leaves showing the Sporonema stage, search 
for the perithecia. note : — 

26. Small hair-like spines protruding from ruptured places in 
the leaf -epidermis. 

27. On the opposite side of the leaf, that a distinct bulge in the 
surface is evident. 

Cut out small bits of the leaves containing perithecia. Mount in water 
and tease away the leaf -tissues, note : — 



54 

28. The shape of the perithecium with its beak. What is 
the function of the beak? draw. 

29. Upon crushing the perithecium, the contents. 

30. Shape and size of asci; thickness of their walls. 

31. Shape and size of ascospores; number in each ascus; septate 
or nonseptate? 

Make drawings of the perithecium, asci and ascospores. (If material 
is not sufficient see Bot. Gaz. 45: 379 and pi. XI.) 

The ascospores are probably forcibly ejected from the perithecia, 
as most ascospores are, and thus they may readily be borne to the unfolding 
leaves and produce primary infection in the spring. That primary infec- 
tion is commonly initiated by ascospores is evidenced by the fact that leaves 
on the lower branches are usually the first to show the disease in the spring. 

From whatever source primary infection may result, a new mycelium 
is started near the veins of the young leaves. As the lesions enlarge 
the acervulus-type of fruit-body is formed. In the prepared sections 
of acervuli, observe : — 

32. The aggregation of hyphas radiating from the base of the 
fruit-body. 

33. The chains of conidia cut-off from the ends of the conidio- 
phores. These form so abundantly that they pile up into balls or strings 
and are held together while dry by a gelatinous matrix. 

34. The individual conidia, as to shape and size. Compare 
with conidia from Myxosporium and Sporonema stages. 

Make drawings to show the structure of an acervulus in cross-section, 
including a portion of the adjacent leaf -tissue and the vegetative hyphae. 
Saprogenesis. When the leaves fall to the ground the mycelium 
spreads rapidly to aU parts of the leaf and lives saprophytically forming 
more acervuli and, in moist situations, the pycnidia of the Sporonema 
stage. The perithecia begin their development after the mycelium has 
existed saprophytically for a time. The ascospores are mature by early 
spring. 

Secondary Cycles originate from the conidia developed in acervuli 
on the leaf- and petiole-lesions. It is suggested by some that the mycelium, 
extending down from the diseased petioles into the twigs, plays an 
important part in the infection of the current year's twigs. This has not 
been proved, but it happens either in this way or by infection during the 
summer resiilting from conidia. The infected twigs are not killed but 
develop normal buds which put out leaves the following spring. However, 
the myceliimi in the twigs gains enough headway during the autumn 
and early spring to cause the death of the twigs at a time when the leaves 
are only partially developed. 

REPORT 

1. From what is known of the life-history of Gnomonia veneta 
(Sacc. and Speg.) Kleb., work out possible control-measures and give 
reasons for each step, based on the life^history of the pathogene. 



BLACK MOT OF CM 

The black rot is an American disease, known as a destructive malady 
in our vineyards for nearly a century before its appearance in Europe in 
1885. It is markedly epiphytotic in its occurrence on cultivated grapes. 

SYMPTOMS 

All green parts of the vine, including the young canes, may exhibit 
lesions of the black rot. Old woody parts are never affected. 

On the leaves. Spots on the leaves are reddish brown and more or 
less circular. The spot at first appears as a small blanched area. The 
spread is concentric but not perfectly circular. Study the material 
before you and observe: — 

1. The size and distribution of the lesions over the surface of the 
leaf. Note the color. 

2. The shape of single spots and their relation to the finer vein- 
lets. Study the margin closely; even or crenulate? 

3. That certain spots show minute black bodies, — structures of 
the pathogene. Are they superficial or sunken in the leaf -tissue ? What is 
their arrangement on the lesion? On which surface are they found? 

Make such sketches as are necessary to bring out the character of the 
spots on the leaves available. (See photograph 1.) 

On the berries. The first signs of the disease on the berries are not 
readily detected except by careful observation. The lesions rapidly 
become apparent, however. Study the diseased berries provided, ob- 
serve : — 

4. The different stages in the development of the lesion from a 
small blanched or brown discoloration to the black and shriveled mummy. 
(See photographs 2 and 3.) 

5. The characteristic black fruit-bodies in the lesion; position 
and arrangement. At what stage in the development of the lesion do 
they appear? (See photographs 3 and 4.) 

DRAW to show the development of the symptoms on the berry. 

On the canes and tendrils. In the material provided, observe : — 

6. The character of the lesions. Compare and contrast with 
those on the leaves; with those on the fruit. 

7. The extent of the lesion on the cane; on the tendril. Are 
the lesions limited and definite? Is there a tendency to girdle? 

8. The number of lesions; their form, size, and the presence of 
the black fruit-bodies of the pathogene. 

Study photograph 5, noting lesions on canes, petioles, midribs and 
tendrils. 

Make drawings to illustrate these studies. 

ETIOLOGY 

The pyrenomycetous fungus, Guignardia Bidwellii (Ellis) Viala and 
Ravaz, is the cause of the black rot of grapes. Its conidial form has been 
given the genus name Phyllosticta, when occurring on the leaves but when 
on the canes, Phoma. A variety of specific names have been applied, 
to it. (See Cornell Bui. 293 : 307.) 

55 



56 

Life-history. This pathogene exhibits in its Hfe-history all the typical 
spore-fomis and pathological phenomena of a large number of the leaf- 
spotting pyrenomycetes. 

Primary Cycles are initiated in late spring and throughout the summer 
(Cornell Bui. 293: 310). The fungus winters largely in the mummies 
which lie on the ground or cling to the vines. (Note in the latter connec- 
tion, photograph 6.) Canes and tendrils may also harbor it. 

Pathogenesis. Some of the mummies are provided for study. 
With a hand-lens, observe : — 

9. The presence of very numerous elevations,— the perithecia of 
Guignardia Bidwellii; the ostiolum at the apex. 

With the hand-lens and needle remove one or more of these perithecia 
(from the soaked mummy provided) to a drop of water on a slide; cover 
and examine with the lower-power, exerting slight pressure on the cover- 
glass. NOTE : — 

10. That if the perithecia are mature, the asci with ascospores 
will be forced out. 

1 1 . The average number of asci in the perithecia ; the number 
of spores in an ascus. 

DRAW an ascus with ascospores. These ascospores constitute in most 
seasons the chief inoculum for primary cycles. Discharged from the 
perithecia during rainy weather, they are blown by the breeze or splashed 
by rain-drops to young leaves or berries where they germinate and the 
germ tubes penetrate the host-tissues, copy Cornell Bui. 293, pi. V, fig. 
28-29. Note the appressorium formed on the end of the germtube. 

Pycnospores overwintered in canes and tendrils may also serve to 
initiate primary cycles. (See Cornell Bui. 293: 326.) With hand-lens 
and needle, remove a few pycnidia from one of the cane-lesions. Crush 
on a slide in water or potassium hydroxide and study, make out : — 

12. The numerous pycnospores set free from the broken pycnidia. 
Their form, size and color as compared with the ascospores. draw. 
These germinate by a simple germtube and infect young leaves and fruits. 
COPY Cornell Bui. 293, pi. V, fig. 34. 

After penetration by the germtube of the ascospore or pycnospore, 
myceliimi is slowly developed in the surrounding tissue. The first evi- 
dences of infection appear in one to three weeks, depending on 
conditions of the host and the weather. After the appearance of the 
lesions, the pycnidia develop (see photographs 1, 3, 4, 7 and 8) and form 
pycnospores with great rapidity. Study prepared slides provided and 
OBSERVE : — 

13. Position of the pycnidiimi in relation to the host-tissues. 

14. The pseudoparenchymatous wall of the fruit -body. 

15. The short conidiophores bearing the pycnospores; their 
origin and arrangement in the pycnidiimi. 

16. The ostioum through which the pycnospores are discharged. 
They are discharged by the pressure resulting from water-absorption 
by the gelatinous substance in which they are surrounded. These initiate 
the secondary cycles. 

Make a drawing to show the structure of the pycnidium and its rela- 
tion to the host-tissues. 

Saprogenesis. The fungus lives saprophytically after the lesion 
is fully developed until the following spring. From July until October 
certain fruit-bodies, known as spermagonia, are developed. They are 



57 

abundant in August and may develop in the same stroma with pycnidia. 
They bear spores called spermatia. (See Cornell Bui. 293, pi. Ill, fig. 19.) 
Their function is unknown but it is believed by some that they are vesti- 
gial male elements. Whether perithecia are produced in the old primary 
lesions is not certain. Pycnospores produced in these primary lesions 
may, however, winter over and new ones may possibly be formed, thus 
completing the primary cycle. (See Cornell Bui. 293: 325-326.) 

The Secondary Cycles, especially those initiated in the late summer 
differ from the primary chiefly in that they certainly give rise to the 
ascogenous structures which develop into perithecia the next spring. 

Pathogenesis. Until the first of August, pycnospores are produced 
in enormous numbers on the mummied berries. They are discharged with 
rain or dew and initiate secondary cycles. Other cycles follow in succes- 
sion as long as pycnospores are developed. After about August 1, 
pycnidial production ceases and instead there are developed in the lesions 
pycnidium-like bodies called pycnosclerotia. 

Study slides provided or Cornell Bui. 293, pi. IV. observe: — 

17. The absence of a central cavity and of spores. 

18. The parenchymatous character of not only the walls but 
of the entire fruit-body. 

19. Differences in the character of the central portion as con- 
trasted with the wall-structure. 

READ Cornell Bui. 293: 329. draw to show the structure of the pycno- 
sclerotium. 

Saprogenesis . With the death and shriveling of the tissues in the 
berry, the fungus continues its activities as a saprophyte. The develop- 
ment of the incipient perithecia (pycnosclerotia) is halted by winter but 
resumed in late spring. In the thin-walled parenchymatous-like center of 
the pycnosclerotium, asci are developed and matured. Study sections 
through mature perithecia and observe : — 

20. The thick dark walls of the cells making up the wall of the 
perithecium; the mouth or ostiolum. 

21. The rather thick-walled asci; their shape, size and position 
in the perithecial cavity. 

22. The ascospores; number and arrangement in the ascus. 
DRAW to show the structure of the perithecium. The maturation 

and discharge of the ascospores completes the secondary cycle. 

Pathological Histology, The effect of Guignardia Bidwellii on the 
tissues of its host is always necrotic. Study sections through leaf-, stem- 
or berry -lesions, observe : — 

23. The contrast between healthy and affected tissues; changes 
in color, and appearance of cell-walls and protoplasts in the diseased 
tissues. 

24. Which tissues in each organ appear to be most affected. 
Is there any evidence of an attempt by the host to stay the progress of the 
pathogene by the formation of cork-tissue about the lesion? 

25. The presence of myceliimi; inter- or intracellular? Are 
haustoria present ? 

DRAW to show the points brought out in the study of the histological 
effects. 

REPORT 

1. Discuss the relation of weather-conditions to the disease, 
keeping remedial measures in mind. 



HARD ROT OF GLADIOLI 

This is one of the destructive diseases of gladioh. No varieties are 
known to be immune to it. The disease appears to occur wherever galdioli 
are grown. So far as known it affects only this host. 

. SYMPTOMS 

Both foHage and corms may be affected. Although it rarely affects 
the foliage of flowering-sized plants, the disease occurs very commonly 
on leaves of plants the first year from seed and cormels. 

On the leaves. Examine the material provided, observe: — 

1. That the affected leaves show more or less circular reddish 
brown spots. 

2. The several zones of color in the lesions. 

3. That the spots usually extend from the edge to the midrib, 
although sometimes the lesion involves the entire width of the leaf. When 
this is the case, that part of the leaf above the lesion soon dies. 

4. That frequently two or more spots coalesce forming a large, 
more or less irregular lesion. 

5. That the spots show black bodies, — the fruit-bodies (pycni- 
dia) of the fungus. 

DRAW an affected leaf showing the several points above indicated. 

On the corms. When gladiolus corms are dug ip the autumn the lesions 
are usually very small. The disease develops during storage until by 
spring many corms are reduced to hard mummies. 

Study the diseased corms provided, observe : — 

6. Size, shape and position of the lesions. Is it possible to 
determine whether or not a corm is diseased without removing the husk 
(sheathing leaf -base) ? 

7. That the center of the lesion is sunken; to what due? 

8. The black water-soaked margin separating diseased from 
healthy tissue. When the disease is not vigorously advancing, this margin 
is absent. 

9. That in old lesions the diseased tissue can be readily chipped 
out with the finger-nail. Cut through a lesion and observe : — 

10. The hardness of the diseased tissue. 

11. The depth of the lesion. 

Make such drawings as are necessary to bring out the above points. 

ETIOLOGY 

The cause of this disease is a fungous pathogene, Septoria Gladioli 
Passerini. 

Life-history. The complete life-histor>^ of this organism has not 
been definitely worked out. 

Primary Cycles. Just how these are initiated is not clear as the follow- 
ing statements indicate. The corms are lifted in the autumn, stored and 
set out the following spring. The fungus continues as an active parasite 
during the winter and is carried to the soil along with the corm. If the 
corm is not too badly affected it will sprout, sending up a shoot and produce 
a corm above with side clusters of cormels. Since in the large majority 

58 



59 

of cases the comis and cormels from a diseased corm are diseased, the 
fungus is thus peq^etuated without the intervention of a spore-stage. 
Just how the fungus passes from the old corm to the new one is unknown. 
It does not grow directly from the one to the other but either works along 
the sheathing leaf-bases or grows out into the soil from whence it at- 
tacks the new corm or cormels. 

The fungus is known to winter in the soil, hence the possibilit}- that soil 
containing mycelium of the pathogene may be splashed by rain onto the 
foliage. The foliage is often beaten down onto the soil where infection 
may occur. The latter is much more likely to occur with the tops of 
seedlings or cormels than of older plants. Why? 

Healthy corms set into infested soil are doubtless commonly infected 
by the pathogene there. 

Pycnospores have been found in the spring in pycnidia on leaves which 
have remained out of doors all winter, but only in very limited numbers. 
These pycnospores will germinate and may constitute the primary inocu- 
lum for the foliage of seedlings or cormels; by the washing of these spores 
into the soil they may infect the corms themselves. 

Pathogenesis. Of the three probable sources of primary inoculiun, 
diseased corms, infested soil or pycnidia in overwintered foliage, the last 
is most readily studied. Examine some of the leaves gathered in the 
autumn, observe: — 

12. The old lesions on the leaves. 

13. The presence of numerous, black elevations, — the pycnidia. 
Is there an ostiolum? draw, showing pycnidia on the leaf. 

With the needle remove several pycnidia to a drop of water on a slide, 
cover and study under the low-power. Pressing lightly on the cover with 
a needle, observe: — 

14. The many pycnospores; size, shape and number of septa. 
The mycelium in the diseased tissue of the corm is not easily detected. 

Make thin sections from the material provided. Stain with methylene blue 
in weak acetic acid for one hour. Wash thoroughly and study, observe : 

15. The large cells still partly packed with starch-grains. 

16. Between them the small blue-stained mycelial threads, 
varying in thickness with the space at their disposal. 

DRAW a few host-cells with the intercellular mycelium. No fruit- 
bodies or spores are known to be produced on diseased corms. 

On infected leaves, pycnidia appear in the lesions simultaneously with, 
or soon after the lesions become evident. Study the pycnidia in section, 
using prepared slides, observe: — 

17. The outer wall of pseudoparenchymatous tissue. 

18. The inner layer of thinner-walled cells from which arise 
the conidiophores. 

19. The conidiophores bearing pycnospores. 

20. The mycelium in the tissue of the host. 

DRAW, showing the structure of a pycindium seen in longisection. 

The pycnospores from the primary lesions on the leaves initiate secon- 
dary cycles. 

Sapr agenesis. The fungus is able to live in the soil without the 
presence of the living host for at least four years. Whether it feeds on 
organic matter and develops in the soil or merely remains alive in a dormant 
condition until corms of gladioli are again available for attack, is not 



60 

known. It grows slowly in culture on agar making a very characteristic 
growth. Examine the plate-cultures provided, observe: — 

21. The form and color of the colonies. The fungus is readily 
obtained in culture by planting in agar bits of diseased tissue from within 
the corm. 

Mount some of the mycelium from the edge of the culture. Study and 
DRAW to show its form and structure. This may possibly be the form in 
which it persists in the soil. 

Secondary cycles. Pycnospores from the primary lesions, blown 
during the summer, fall upon healthy leaves where, under proper conditions 
of temperature and moisture, they germinate and produce secondary 
infection. From the large percentage of diseased corms formed by plants 
growing from seed and cormels, the foliage of which was attacked by 
Septoria Gladioli Passer., it appears that secondary infections also occur 
from spores washed by rain down into the soil, where they germinate and 
infect the corm. 

So far as known the secondary cycles duplicate in details the primary 
cycle. 

Pathological Histology. The lesions of this disease are strictly necrotic. 
The pathogene promptly kills the protoplasts of the invaded tissues. 

Study prepared sections through a lesion on the leaf, observe: — 

22. The collapsed and shriveled cells in the diseased portion. 
Compare with the cells in the healthy region. What tissues are affected? 

23. The effects on the cell-organs, i. e. nucleus and chloroplasts. 

24. The mycelium of the pathogene; its relation to the host- 
cells. 

25. Position and relation of the pycnidia to the host -tissues. 
DRAW to show conditions and structures of the fungus in the diseased 

leaf. 

Study sections through a lesion in a corm. observe :— 

26. The absence or the small amount of starch in the diseased 
as compared with that in the healthy tissue. 

27. That many cells in the diseased area are collapsed; due 
to what ? 

28. The layer of cork-cambium laid down between healthy and 
diseased tissue. May this be associated with the fact that the diseased 
tissue can be chipped out with the finger-nail? 

29. The presence of intercellular mycelium. 

DRAW to bring out these points, showing both healthy and diseased 
tissue. 

REPORT 

1. A grower plants annually over one hundred acres of gladioli. 
About fifty per cent of his corms are diseased. Outline a scheme, bringing 
in the principles of eradication and protection, whereby he can gradually 
work from diseased to healthy stock, probably requiring ten years or 
longer. Indicate the difficulties to be encountered in carrying out such a 
scheme. 



STMAWBERMY LEAF=SPOT 

There are at least two leaf-spot diseases of strawberries, but the one 
to which this name is applied is by far the most prevalent and injurious. 

SYMPTOMS 

The lesions on the leaves are most numerous and conspicuous, but the 
petioles and fruit-pedicles are often affected. In the material provided, 
OBSERVE : — 

1. The spots on the upper side of the leaf, ranging in size from 
mere pink flecks to areas almost one centimeter in diameter. The large 
spots always have white centers and pink or brick-red margins. 

2. That some of the spots have coalesced to form larger lesions 
which may cover a large part of the leaf-area. 

3. The character of the under surface of the spots as compared 
with the upper surface. To what extent are the veins affected? 

4. The elongated spots on the leaf-petioles and on the fruit- 
pedicles; the white centers of the lesions, which are very conspicuous. 
Are they surrounded by red margins as in the leaf? 

5. The small and dried berries on the diseased pedicles. One 
or two berries may develop normally before the food supply is cut off, 
after which the others shrivel and are worthless. 

Make drawings to show the symptoms on the different parts of the 
host. 

Compare these symptoms with those of the Marssonia leaf-spot on 
strawberr}^ 

ETIOLOGY 

The pathogene causing this disease is an ascomycetous fungus, Mycos- 
phaerella Fragariae (Tulasne) Lindau, belonging to the family Mycos- 
phaerellaceae of the order Sphaeriales. 
Life-history. 

The Primary Cycle is initiated by the ascospores of the sexual stage 
found in the dead leaves on the ground. These ascospores, while still in 
the fruit-body, germinate sending out a long germ tube which pushes out 
through the mouth of the perithecium and produces a conidium at the tip. 
(See Cornell Bui, 15: 178-180.) Make a diagrammatic drawing to show 
production of primary conidia. 

Pathogenesis. The conidium is blown to a young leaf where it pro- 
duces a germtube that directly penetrates the upper epidermis and invades 
the tissues beneath. 

Cut out a diseased spot with the scalpel, and include a margin of the 
healthy tissue. Place the small piece in the casserole with a dropperfull 
of potassitim hydroxide solution, and boil over a microbumer for half a 
minute. Rinse two or three times with water, then mount and observe : — 
G. The browned or blackened appearance of the diseased area 
in contrast to that of the healthy tissue immediately adjoining it. 

7. The mycelium ramifying all the diseased tissue; its color, 
width, septation and branching. Does it extend into the tissues which 
still contain chloroph}-!? 

61 



62 

Make a drawing of the tissue as seen in this mount, showing the my- 
celium and the extent of its penetration. 

The hyphae invade the cell, using up the contents, and permit the air 
to enter the tissue, thus giving the white appearance to the center of the 
lesion. These hyphal threads grow upward and produce a stroma just 
below the cuticle; the cuticle is then ruptured permitting the fruit-stalks 
to emerge. 

With the aid of pith, cut thin sections through the diseased area of the 
leaf. Mount these in water (or use prepared slides) and observe : — 

8. The location of the mycelium and its relation to the diseased 
cells. 

9. The mat of myceliimi or stroma just below the cuticle. 

10. The three or more short stalks or conidiophores which arise 
from each stroma. Are they septate? A single conidiimi is borne on the 
tip of each conidiophore. The conidia will probablv not be present in 
this mount. Why? 

Make a drawing of a cross-section of a leaf showing the location and 
structure of the stromata with their conidiophores. 

For study of the conidia, scrape several of the spots on the leaf with 
the scalpel and make mounts in potassium hvdroxide. (If spores are not 
present, consult text books of Duggar, Stevens, or Tubeuf and Smith.) 
observe : — 

11. Their size, shape and septation. draw. 

Sapro genesis. The infected leaves which drop to the ground 
are still farther invaded by the mycelium of the pathogene. Thi s myceliimi 
finally develops a sexual fruit-body or perithecium. This is at first covered 
by the tissue of the leaf, but soon breaks through the epidermis. 
In the leaves provided observe :— 

12. Small black bodies just large enough to be seen with the 
naked eye, near or in the old lesions. They are never numerous. Make 
a drawing of part of a leaf showing these small black perithecia. Com- 
pare their size and appearance with the acervaili of Marssonia found on 
old fallen leaves of strawberry. (The two fungi are not related.) How 
can the perithecium be distinguished from the acervulus? 

With the aid of pith, cut thin cross-sections of a leaf containing perithe- 
cia (or use prepared slides). Under the microscope, observe: — 

13. That the perithecia are more or less hemispherical. 

14. That the wall of the perithecium seems to be made of two 
layers, the outer one being made up of thick-walled pseudoparenchymatous 
cells, and the inner one of thin-walled cells. 

15. That there is an opening at the tip of each perithecium, — the 
ostiolum. 

16. That arising from the inner base of the perithecium are 
club-shaped bodies, — the asci containing eight hyaline, uniseptate spores 
with acute ends. 

Make a drawing of a cross-section of the perithecium, with the sur- 
rounding host-tissue. Make an enlarged drawing of a single ascus with 
its spores. 

It is from these spores that the primary cycles start, the following spring. 
The mycelitim may also live during the winter in the partially green leaves 
and produce conidia the following spring, thus initiating primary cycles. 



63 

This is probably quite as often the means of wintering over as that 
afforded by the perithecial stage. 

The Secondary Cycles originate from the conidia on the young 
diseased leaves. These conidia infect the petioles, pedicels and other 
leaves. The pathogene completes its development in the secondary cycles 
in the same manner as in the primary. 

REPORT 

1. Illustrate in cartoon-like drawings the life-history of Myco- 
spkaerella Fragariae (Tul.) Lindau. 



PSEUDOPEZIZA LEAF=§POT OF ALFALFA 
AND CLOVER 

This is an enphytotic disease of alfalfa and clover in most regions 
where these crops are grown. It annually reduces the yield, especially 
of alfalfa but rarely, if ever, kills out the plants. 

SYMPTOMS 

The lesions of this disease are confined to the leaves and stems. They 
are much less common and of little importance on the latter. 

On the leaves. Study the specimens provided and observe : — 

1. The spots on the alfalfa leaf . Do they show on both surfaces? 
Note their size, shape, color and position on the leaf. Compare with 
those on the clover leaf. 

2. That these are usually surrotmded by a yellowish zone. 
Note that this is more pronounced between several adjacent spots. 

3. The darker, raised center of the spot made up of the fruit- 
body of the pathogene. Is this visible on both sides of the lesion? 

Make dr.\ wings to show the character of the lesions on both clover 
and alfalfa leaves. 

With the hand-lens or with the low-power of the microscope examine 
several spots which show the fruit-bodies of the pathogene. note : — 

4. The radially split crater-like openings and the waxy-appearing 
surface within. These fruit-bodies at the center of the spots serv^e to 
definitely distinguish the Pseudopeziza spot from other leaf -spots common 
on alfalfa and clover. 

DRAW to show the appearance of the fruit-body under hand-lens or low- 
power. 

5. A badly diseased leaf in comparison with one which has but 
few spots. Is there any difference in the general color? Does a single 
spot ever involve the entire leaf; if not, how is the leaf killed? 

6. The passepartout provided and determine where on the plant 
the most serioush' spotted leaves occur, and what are the tdtimate results. 
Compare photographs 1 and 2. 

Make sketches to show the general effects on the alfalfa and the clover. 

ETIOLOGY 

The cause of this leaf -spot is a discomycetous fungus, Pseudopeziza 
Medicaginis (Libert) Saccardo. The pathogene on the clover is regarded 
by some as a distinct species and goes under the name, Pseudopeziza 
Trifolii (Bivona-Bemardi) Fuckel. They are probably identical. 

Life-history. But one kind of spores, the ascospores, are certainly 

known to be produced by this fungus. As repeated crops of these are 

produced throughout the season, the only distinction between primary 

and secondan.^ cycles is that the former are those first initiated in the spring. 

Primary Cycle. 

Pathogenesis. The sources of the primary inoculum are the 
apothecia produced in earty spring on overwintered living leaves. These 
leaves become infected late in the autumn but complete development, 
including apothecial formation, is halted until spring when with the arrival 

64 



65 

of warm weather, the apothecia are developed on these ovenA'intered 
lesions. It has been held by some that from the lesions on fallen leaves 
which have laid on the ground over winter, apothecia are produced which 
supply the primary inoculvmi. 

Crush a mature apothecium (one that is open) from leaves provided, 
tease well apart, cover and study: — 

7. The mature ascospores in the ascus; their shape, structure, 
color, and niimber in the ascus. dr.\w. 

These spores constitute the priman^ inocultun. If germinating asco- 
spores are available, study and sketch. 

The germtube penetrates the upper epidermis of the leaf, and produces a 
locally spreading miycelium with a resulting spot or lesion. At the center 
of this is shortly developed an apothecium without any intermediate 
production of asexual spores as is usually the case among the ascomycetes. 

Make freehand sections of the leaves through the spots, stain with 
methyl blue, wash, and examine with the high-power of your microscope. 
observe: — 

8. The brown host-cells; the character of their contents; 
the relation of the mycelium to these cells, inter- or intracellular? 

9. How far the mycelium extends. What causes the yellowing 
around the brown cells? 

10. The blue-stained structure on one side of the brown-spot, — 
the apotheciimi. On which surface does it occur? How deep into the 
leaf does it extend? How does it become exposed? 

11. That the lower part of the apothecium, — the stroma, is 
composed of sm.all cells. 

12. That arising from the stroma is the hymenium. Of what 
is it composed ? (It may be easier to observe the structure of the hymenium 
by crushing a few of the older spots in a drop of water.) 

13. Within the asci, the oval ascospores; how many? Note 
the oil-drops or guttulae at each end. These spores are shot from the asci 
during moist conditions, thus disseminating the pathogene. 

Make dr.\wings, (a) to show the relation of the mycelium to the host- 
cells, its extent and the apothecium; (b) an enlarged view of the hymenium 
to show its structure, the spores and their arrangement in the ascus. 

Saprogenesis. There is some evidence that apothecia may be 
formed on spots which were imm.ature when the leaves fell to the ground. 
This, together with the fact that apothecia may be produced in artificial 
culture m.edia, indicates that apothccial development may in some cases 
restilt in nature from saprophytic activity. Since, however, mature 
ascospores in apothecia on living leaves are always comjmon and abundant, 
saprogenesis, as such, is an unimportant if not uncommon phase of the life- 
cycle. 

Secondary cycles. These are initiated by ascospores from the 
apothecia produced during the primary cycle, and the pathogene repeats 
in all details the structures and activities of the primary cycle. 

REPORT 

1. Explain the effects of this fungus on the leaf and on the yield. 

2. Show graphically the life-history of Pseudopeziza Medicaginis 
(Lib.) Sacc. 



(CEECOSPORA LEAF^SPOT OF BEETS 

Although this disease is found more often on sugar-beets, it also affects 
the common garden-beet. The disease is typical of a great many Cerco- 
spora leaf-spots on different hosts. 

SYMPTOMS 

Lesions are usually present only on the leaves, though they have been 
reported on the petioles, the bracts, peduncles of the flowers and on the 
seed-pods. 

In the material provided, observe : — 

1. The variously sized spots with brown or grayish centers 
surrounded by a zone of red or purple (brown in dried specimens) ; their 
shape and distribution over the leaf; differences in the character of the 
spot on the upper and the lower surface. Make drawings to show these 
points. 

As the diseased leaves drop off, new ones are formed, thus producing 
an elongated crown on the root. (See photographs or Duggar, Fimgous 
Diseases of Plants, p. 310.) 

ETIOLOGY 
The pathogene responsible for this disease is Cercospora Beticola 
Saccardo, one of the Hyphomycetes. Only the asexual form of the fungus 
is known. 
Life-history. 

The Primary Cycle is initiated by conidia developed on overwintered 
tops and crowns left in the field. They are blown or spattered by the rain 
to the first young leaves. 

Pathogenesis. Scrape several of the spots with a scalpel and make 
a mount in water of the material thus procured. Under the microscope, 
OBSERVE :— 

2. The thin, long, needle-like spores, which are slightly thickened 
at one end and tapering at the other. How many septa in each? draw. 

These conidia germinate and the germtubes enter the leaf through 
the stomata. Study the germinating spores, observe: — 

3. The number of germtubes developed from each spore; num- 
ber from each cell. draw. Study leaf -invasion through the stomata, 
as shown by Pool and McKay, Jour. Agr. Research 5, pi. LXXXI. copy 
one or more of the figures, A, B, C, or F. Label in detail. 

The mycelium developed from these germtubes spreads through and 
kills the tissue. A leaf -lesion or spot thus results. Very shortly this 
mycelium develops the fruiting structures on the surface of the lesion. 

Cut out a spot from the leaf including a margin of healthy tissue. 
Macerate in a casserole for half a minute in potassium hydroxide over a 
microbiimer. Mount in water and observe : — 

4. At the edges of the lesion, the almost hyaline branching 
mycelium, which may be uniform in width or have occasional cells thick- 
ened, with granular vacuolated contents, draw. 

5. The tufts of brown conidiophores closely crowded at the base 
and widely separated at the top. 

6. That very few of the conidiophores are straight but appear 
knotted or angled. 

66 



67 

7. That at each angle is a scar where a conidium had been 
borne. How many scars does each conidiophore have? 

S. The sclerotia-like mass of cells from which the conidiophores 
arise; color, size. 

Place a lesion, lower surface up, under the binocular microscope and 
OBSERVE : — 

9. The points emphasized in 5 and 6. 

10. The relation of the tufts of conidiophores to the host- 
tissue, especially to the stomata. 

11. The attachment of the conidium to the conidiophore. Is 
the thickened or the thin end attached to the stalk? 

Make a drawing showing the points brought out under paragraphs 
4 to 8. 

Saprogenesis. No perfect stage of this fungus has yet been found. 
The fungus continues to fruit for a time after the leaf -tissues die. The 
sclerotia-like bodies from which the conidiophores arise remain alive under 
favorable conditions, in the host-debris. They produce a new crop of 
conidia the next spring which gives rise to the primary infections. 

Secondary Cycles are started repeatedly during the summer by 
conidia from primary and secondary lesions. They repeat in detail the 
phenomena of the primary cycle. 

REPORT 

Review and abstract one of the following papers by V. W. Pool and 
M. B. McKay. 

Climatic conditions as related to Cercospora beticola. Jour. 
Agr. Research 6:21-60. 1916. 

Relation of stomatal movement to infection by Cercospora 
beticola. Jour. Agr. Research 5: 1011-1038. 1916. 



SEPTOMIA LEAF=§POT OF CELERY 

This disease is often called late blight in contradistinction to the 
early blight or Cercospora leaf-spot. Both of them will affect the plant 
while it is still in the seedling-stage, but the Septoria leaf-spot is usually 
most severe later in the season. Both diseases are very destructive to 
the celery-crop. 

SYMPTOMS 

This disease is found on all parts of the plant above the surface of the 
soil. 

On the leaves. Examine specimens of both dried leaves and those 
preserved in liquid, observe: — 

1 . The numerous spots scattered over the leaf, both on the upper 
and lower surface. Do the lesions extend through the leaf? 

2. That the spots are irregular in outline and often coalesce 
with neighboring spots. Do the spots have an}^ definite zones surrounding 
them as found in lesions on strawberry or beet leaves ? 

3. That each spot is marked with a large number of black 
papilla-like bodies, — the fruit-bodies of the pathogene. 

Make a drawing of a diseased leaf. 

On the stems provided, observe : — 

4. The small brown sunken areas, which also show the black 
fruit-bodies, draw. 

On the seed. If specimens of diseased seed-capsules are not provided, 
see Zeitschrift fur Pflanzenkrankheiten 20:7. 1910; or Michigan Special bul. 
77:4. COPY. 

ETIOLOGY 

The pathogene is Septoria Petroselina Desmazieres var. Apii Briosi 
and Cavara. 

Life-history. Only the conidial form of the pathogene is known. 
In this form it is able to sundve the winter, hence a sexual stage, if it does 
exist, is probably of little importance in the perpetuation of the fungus. 

The Primary Cycle has its beginning with infection by the conidia 
which are present in the debris of last-years crop, or with those which 
are found on the seed. 

Pathogenesis. Crush in water several of the small black pycnidia 
from old leaves, cover and observe : — ■ 

5. The thin long cylindrical spores, slightly curved and tri- 
septate. (Staining with iodine may be necessary to make the septation 
evident.) 

These spores are blown about or spattered to the outer, lowest leaves 
of the celery, where they germinate, sending germtubes through either the 
upper or lower epidermis. The mycelium which is usually intercellular, 
soon forms pycnidia. 

Cut thin cross-sections of a diseased spot or use prepared slides and 
under the high-power, observe : — 

6. The large thick-walled pycnidium, which ordinarily is nearly 
globose but in the sections may be flattened or dented. Does it set 
deeply into the tissues? 

68 



69 

7. The pseudoparenchymatous nature of the pycnidial wall, 
and the round opening, — the ostiolum. 

8. Lining the pycnidial cavity, the short, sharply pointed 
conidiophores on the ends of which the slender conidia are borne. 

9. The mycelial threads penetrating the tissue all about the 
pycnidium; the septation, contents and the branching of this mycelium. 

Make an enlarged drawing of a pycnidium with the surrounding 
host-tissue. 

Saprogenesis. There is no vegetative activity during the sapro- 
genic phase. The fungus winters over as spores in the pycnidia in the old 
leaves and stems which have been left in the field. These spores serve as 
the inoculum the following spring. 

Secondary Cycles. The conidia from the diseased leaves ooze 
from the pycnidium whenever moisture is present and are transfered by 
rain, tools or animals to the stems or other leaves. Secondary infections 
are repeated until the host is killed or removed. 

REPORT 

1. Make a list of all the Septoria leaf-spot diseases found on 
vegetables, to which you can find references in available literature. Give 
both the name of the pathogene and of the host. 



THE BLACK SPOT OF MOSES 

The black spot is a disease peculiar to the rose, and appears wherever 
this ornamental is grown, in temperate and tropical regions, in greenhouse 
and garden. It is, next to the powdery mildew, the most serious disease 
of roses. Few varieties appear to be immune. 

SYMPTOMS 

The lesions of this disease are confined to the leaves. Examine the 
material provided, observe: — 

1. That some of the affected leaflets show isolated, more or less 
circular black spots. 

2. That the margins of these spots are radiate-fibrillose. Study 
with the hand-lens. 

3. The distribution of the lesions. Do they occur on both sides 
of the leaflet ? Are they marginal or located more toward the center of the 
blade? 

DRAW a leaf showing the character of the isolated black spots. 

4. That some leaflets are nearly or quite covered by the lesions. 
Explain how this combes about. 

5. That the lesions vary somewhat on different varieties of 
roses. In what respects ? Show this in sketches or in notes. 

The leaves of some varieties become yellow very soon after the spots 
develop. In others there is little or no yellowing but in either case pre- 
mature defoliation occurs. The early loss of foliage weakens the plants. 

ETIOLOGY 

The cause of this disease is an ascomycetous pathogene, Diplocarpon 
Rosae (Libert) Wolf, the conidial form of which has long been known under 
the name of Actinonema Rosae (Libert) Fries. 

Life-history. The life-activities of this pathogene are perfectly cor- 
related with the seasonal changes of the temperate zone, although the 
family, Microthyriaceae, to which this fungus belongs, is largely tropical. 
The Primary Cycles are initiated in the spring by inoculum from the 
overwintered infested leaves on the ground. 

Pathogenesis. The inoculum consists of ascospores produced in 
fruit-bodies in the fallen leaves. Remove a bit of tissue containing some 
of these minute fruit-bodies, crush, and under the microscope, observe: — 

6. The two-celled ascospores, some floating free, others stiU 
within the asci. draw or if material is not available copy from Bot. Gaz, 
54, pi. XIII, fig. 16-17. 

The ascospores do not appear to be shot into the air but simply ooze 
out on the .surface of the old leaf. How may they reach the unfolding 
leaves of the plant ? These ascospores will germinate only on the surface 
of living rose leaves, copy Bot. Gaz. 54, pi. XIII, fig. 18. 

The germtube penetrates the cuticle and gives rise to hyphae which, 
spreading just beneath it over the epidermal cells, form radiating strands 
of subcuticular mycelium. 

Examine one of the spots on the leaves provided and make out : — 

7. These fine branched radiating mycelial strands. What 
color are they? 

70 



71 

To study the structure and arrangement of these subcuticular strands, 
use prepared stained surface-mounts or make such mounts under direction 
of the instructor. Some mycelial threads penetrate throughout the leaf- 
tissue beneath. At certain places on the subcuticular mycelium, the 
conidial fruit -bodies are developed. 

Again examining one of the old dark spots with the hand-lens, make 
out: — 

8. The conidial fruit-bodies, minute black specks, — the acerv^tili. 
Make an enlarged diagrammatic drawing to show surface view of the 

subcuticular mycelium and acervuli in their proper relation to each other 
and to the leaf. 

Make thin cross-sections through a spot showing numerous acervuli. 
Stain with methylene blue, wash, cover and observe : — 

9. The relation of the acervuli to the leaf -tissues ; subepidermal, 
subcuticular, or on the surface? (Compare with prepared stained slides.) 

10. Structure of acervidus, conidiophores and conidia. 

11. Mycelium, in epidermal cells and palisade-tissues, beneath 
the acervulus; inter- or intracellular? 

Make a detailed drawing of a section through the acervulus. 

These conidia, either borne by the wind or splashed by rain to healthy 
leaves, germinate to initiate the secondary cycles. Study germinating 
conidia and draw; or copy Bot. Gaz. 54, pi. XIII, fig. 3. 

Saprogenesis. After the diseased leaves fall to the ground, the 
subciiticular mycelium forms peculiar shield-like structures, beneath 
which in the leaf-tissue below the epidermal cells, the mycelium fonns a 
peritheciiun. These perithecia begin to form in the autumn but do not 
mature until spring. 

If prepared sections are available, study and work out in a drawing, 
the structure of the perithecium, or copy figures from Wolf's article in 
Bot. Gaz. 54, pi. XIII. 

Secondary Cycles. These are initiated repeatedly throughout the 
season by conidia from acervuli. Leaves involved in the secondary 
cycles fall prematurely like those suffering from the primary infections 
and the next spring also produce a crop of ascospores. 

Pathological Histology. The brown or black coloration of the spots 
is due to the necrosis of the epidermal and palisade-cells and not to the 
mycelium as one might expect. 

12. If fresh leaves are available, make thin cross-sections 
through a spot and verify the above statement. 

SKETCH in detail a few diseased and healthy cells to show effects on the 
protoplasm and ceU-organs. 

report 

1. Detail in a letter to a grower of outdoor-roses two methods 
for the control of this disease, one an eradication-method, the other a pro- 
tection-method and show how one may be made to supplement and streng- 
then the other. Assume that the grower knows the facts of the life- 
history of the pathogene. 



LEAF^SPOT DISEASES 

There are a large number of diseases affecting crops which may be 
designated under the term leaf -spot diseases. In most cases little m.ore 
is known than the names of the fungi a,ssociated with the lesions. They 
are, nevertheless, in the aggregate, of considerable economic importance 
and deserv^e more careful and exhaustive study. 

These diseases take their names from the character of the lesions most 
commonly exhibited, namely, spots on the leaves. These spots vary 
in size from mere specks to extensive dead or discolored areas, often involv- 
ing a large part of the blade. In most cases they are rather definite in 
outline, more or less circular, and necrotic in character. They may, 
however, be angular or linear. They usually exhibit fruiting stnictures 
of the pathogene either on the upper or lower surface near the center of 
the lesion. Most of these leaf -spots are due to ascomycetous pathogenes 
which produce during pathogenesis the conidial or imperfect fruiting 
structures. Hence one may expect to find on these spots either conidio- 
phores, acerviili or pycnidia. In some cases the fruit-bodies of the sexual 
stage are formed in the lesions. For many of these leaf-spot fungi, the 
sexual or perfect stage is unknown; in many cases because it has never 
been sought for. It will usually be found to develop during saprogenesis 
on the old fallen leaves, either in the areas of the original spots or scattered 
over the dead foliage. 

SYMPTOMS 

Specimens of several leaf-spot diseases have been provided. Study 
the symptoms exhibited by each and make drawings to show the same. 
Note that the opposite sides of the lesion may differ. The drawings and 
labeling should bring out the different color-zones in the lesion, size, dis- 
tribution in the blade and other characters. 

ETIOLOGY 

The pathogene in the material provided is in the pathogenic phase of 
either the primary or secondary cycle. Remembering that it is probably 
an ascomycetous pathogene, search carefully for the characteristic fruiting 
stnictures of ascomycetes. Use your hand-lens or place the leaf on the 
microscope-stage under low-power and examine with reflected light. 

If conidiophores are found, wet the scalpel, scrape some of 
the conidiophores from the spot, and mount in potassitun hydroxide. 
Study and draw conidiophores and conidia. How are the conidia 
borne on the conidiophores? 

If acerv'uli or pycnidia are found, make an enlarged sketch 
of the lesion as it appears under the hand-lens, showing location and 
distribution of these fruit-bodies. 

To study the internal structures of these fruit-bodies and their relation 
to the host-tissue, make thin cross-sections from dry m.aterial and mount 
in potassium hydroxide, or from alcoholic material, in water. Show the 
character of these structures and relations in drawings. 

Determine the name of the pathogene by using Duggar, Fungous 
Diseases of Plants; Stevens, Fungi which Cause Plant Disease; Cooke, 

72 



73 

Fungoid Pests of Cultivated Plants; Hartig, Diseases of Trees; and other 
books which the instructor may suggest. Study in this way as many as 
you can of the leaf-spot diseases provided. 

REPORT 

1. Drawings fully labeled, giving the name of each disease 
and the pathogene in each case. 

2. Consult the available literature on two of the diseases studied 
and suggest possible methods of control. 



BITTER MOT OF APPLES 

Although bitter rot is known to occur on many different plants, it is of 
preeminent importance on the apple in the southern and central states. 
It is practically unknown in northeastern United States. It has been 
estimated that American growers of this fruit lose from bitter rot in some 
years ten million dollars. For the most part the disease has been called 
bitter rot, but the nam.es anthracnose and ripe rot have been used. 

SYMPTOMS 

The disease affects the fruit and the bark of limbs and twigs. Leaves 
never show the lesions. 

On the fruit. Spots on the apple ordinarily appear in July and August, 
although they ma}^ be seen as early as June and as late as October. Lesions 
at first appear as a light-brown discoloration beneath the skin. They are 
very small but soon enlarge and become firm in texture and circular in 
outline. When the lesions are three millimeters or more in diameter, 
the surface is distinctly sunken. Usually each fruit shows only a few 
spots, but in cases of severe infection more than a thousand separate 
lesions may occur. In cases like the latter very few if any of the spots 
increase in size, but instead, after reaching a diameter of about two milli- 
meters, cease enlarging and appear as brown raised blisters. Study the 
specimen in the jar and observe: — 

1. The stage of development of the affected apple. At what 
stage, green or ripe, was the fruit affected? Make an outline drawing 
of the apple. 

2. The spots on the fruit; their size, color, distribution, appear- 
ance (sunken or raised). Estimate the ntmiber. draw lesions on the 
apple already outlined. Have any of the many spots enlarged? If so, 
show relative size in the sketch. See illustration photograph and Jour. 
Agr. Research 4, pi. VII, fig. 1. 

In more typical cases of bitter rot only a few spots occur on each fruit. 
(See PI. Ind. Bur. Bui. 93, pi. I and VI.) These spread rapidly, involving 
the entire fruit in a short titne. Ripe apples have been inoculated with 
pure cultures of the fungus and placed in an incubator at 30° C. One or 
more specimens are provided for each student. Study the apples with the 
following POINTS as a guide. 

3. The size of the spot; its form, and color. Note any other 
characters exhibited. 

4. Do you find any evidence of the fruiting bodies of the patho- 
gene on the surface of the lesions? If so, note their distribution, color, 
and relation to the tissues of the apple (sunken or superficial?). 

5. Study the following figures so far as available: — PI. Ind. Bur. 
Btd. 44, pi. I, II, IV, and VI; PI. Ind. Bur. Bui. 93, pi. I and VI; Illinois 
Bid. 118, pi. Ill and VIII. Read the legend in each case. 

Make a drawing to bring out the points observed, copy a figure 
such as is shown in PI. Ind. Bur. Btd. 44, pi. II. 

Cut the artificially infected apple longitudinally into halves (do not use 
the razor) note: — 

6. Taste of the rotten flesh. Is it bitter? Examine PI. Ind. Bur. 
Bui. 44:17, fig. 2-3. Make a drawing to show the lesion internally, its 
depth and shape. 

74 



75 

Affected fruits may fall from the tree during all stages of the disease. 
Explain why. Whether or not they fall, there finally results a much- 
wrinkled dark-brown mummy. Examine the mummies provided and 
PI. Ind. Bur. Bui. 93, pi. I, VI and VII. Read the descriptions, sketch 
to show a fallen and a hanging mummy. 

On the limbs. The lesions on the limbs are cankers. Twigs are wholly 
killed and are then said to be blighted. Examine the canker before you 
and OBSERVE : — 

7. The general appearance of the lesions as to color, size, shape 
and surface characters. Do vou find evidence of a dead twig at the center? 
(See PI. Ind. Bur. Bui. 44, pi. VII and Illinois Bui. 1 18, pi. I and IV.) Make 
a DRAWING of a typical canker. Young cankers begin as small, dark, 
sunken, sharply defined areas. (See PI. Ind. Bur. Bui. 44, pi. IX.) 

ETIOLOGY 

The pathogene, now known as Glomerella cingulata (Stoneman) 
Spaulding and von Schrenl<, is an ascomycetous fungus belonging to 
the family Gnomoniaceae of the Pyrenomycetes. Its conidial stage has 
long been known under several names, that usually applied to the fungus 
on the fruit being Gloeosporium fructigenum Berkley. 

Life-history. Extensive studies on the life-history of this fungus 
have been made, especially by American workers, so that the details are 
now well known. 

The Primary Cycle is very definite. There are many secondary ones 
during the season. 

Pathogenesis. The mummied fruits hanging on the trees and the 
cankers are the sources of the primary inoculum. The fungus hibernates 
in these, producing during the spring conidia in acervuli. Apparently 
ascospores, which may initiate the primary cycle, ordinarily play but a 
small role in the life-history of this pathogene. Make crushed mounts 
of acervuli from the mummies or cankers provided and observe : — 

8. The hyaline conidia; their form, granular contents, abundance 
and variation in size, draw several to show these points. 

These conidia, washed and splashed by the rains to young fruits, initiate 
primary infection. Study the germinating spores provided, observe: — 

9. The long septate germtubes; point of origin. 

10. The dark thick-walled appressoria formed at the tips of some 
of the germtubes. What is their function? (Compare PI. Ind. Bur. Bui. 
44, pi. V.) DRAW several germinating spores. 

The germtubes, directly from the spores or from the appressoria, 
penetrate the uninjured skin of the apple or gain an entrance through 
v/ounds and a mycelium is rapidly developed resulting, usually within a few 
days, in a distinct rot-lesion. Fruits generally affected are often in a 
conical region beneath a miunmy or canker. Remove a bit of the diseased 
apple flesh with the forceps and study: — 

11. The mycelium; its character and branching, draw. 
The lesions soon begin to show developing fruit-bodies, — the acervuli 

of the conidial stage of the fungus. 

Study the diseased fruits provided, note : — 

12. The location and concentric arrangement of the acervuli 
within the lesion. 

13. The different degrees of development of these fruit-bodies 
from the center toward the margin of the spot. 



76 

14. The mass of conidia oozing from each mature acervulus; 
color and consistency of the mass. Remove a spore-mass to a drop of 
water; note the dissolving effect. The conidia are held together by a gela- 
tinous substance which quicklv dissolves in water. (See illustration 
photograph; PI. Ind. Bur. Bui. 93, pi. VI, fig. lA; Illinois Bui. 118, pi. 
Ill, fig. 2 and pi. V, fig. 1; PI. Ind. Bur. Bui. 44, pi.- 1, II and VI.) These 
conidia initiate secondary cycles. 

Make a careful drawing of several acervuli as they appear under the 
hand-lens. 

Make a number of very thin sections through a bit of the affected tissue 
showing mature acervuli (or use prepared sections) . Study and observe : — 

15. The great mass of conidia pushed forth through the rup- 
tured epidermis, the recurved edges of which show at each side of the 
spore-mass. 

16. The mycelial bed, forming the base of the acervulus, from 
which the short simple conidiophores arise. How are the conidia formed? 

17. The mycelium ramifying the adjoining host-tissues; inter- 
or intracellular ? 

DRAW to show the structure of the acervulus and its relation to the host- 
tissues. 

Saprogenesis. With the mummification of the fruit and the death 
of the bark in the cankers, the mycelium does not die but sometimes 
continues a saprophytic existence. It lies dormant through the winter, 
resimies its activities in the spring and usually produces acervuli, though 
sometimes the sexual fruit -bodies, perithecia, are produced. The latter 
are rare. Acervuli having been studied, a brief examination of the perithe- 
cium may be made. Study the prepared sections, observe : — 

18. The long blunt protuding beak of the perithecium. 

19. The relation of the perithecium to the host-tissue. 

20. The asci; form and arrangement in the peritheciimi. 

21. The structure of the perithecial walls. 

22. The ascospores ; number and arrangement in an ascus ; form. 
DRAW a longisection of the perithecium. The ascospores, when pro- 
duced, doubtless serve to initiate primary cycles. 

Secondary Cycles are initiated repeatedly throughout the growing- 
season on fruits and twigs, causing fruit-rot, twig-blight and limb-cankers. 
The pathogene passes through the sam.e forms and phenomena as in the 
primary cycles. 

Pathological Histology. Slides are provided showing sections through 
young spots such as are to be observed on the fruits in the jar. Study the 
sections and draw to show pathological effects observed. Explain the 
formation of the cork-layer. 

report 
1. Make a diagram illustrating the cycles in the life-history of 
Glomerella cingulata (Stonem.) S. and von S. 



APPLE BLOTCH 

This disease is generally known as apple blotch, but it is sometimes 
called black scab, fruit-blotch, and star-fungus. 

It should be clearly borne in mind that apple blotch is entirely distinct 
from sooty blotch and apple scab, in spite of the common names applied. 
This disease, like bitter rot, is confined largely to the central and southern 
states, and in the southern half of the apple-belt it is more destructive, 
taken year by year, than bitter rot and apple scab combined. 

SYMPTOMS 
The fruit, bark and foliage are all affected by this disease. 
On the fruit. The lesions vary somewhat with the variety. Using 

available affected fruits, and PI. Ind. Bur. Bui. 144, pi. I, II, IV, V and 

VI, study the symptoms and observe: — 

1. That the first evidence of the disease is a very small in- 
conspicuous, light-brown speck, which, under slight magnification, has the 
appearance of a stellate collection of brown fibers just beneath the epider- 
mis. On account of this stellate character of the spot this disease is at 
times confused with apple scab. On young apples, the lesions are at 
first water-soaked, and in wet weather there may be a yellowish gummy 
exudate therefrom. 

2. That spots enlarge radially, attaining a diameter of from 
3-10 or more millimeters, becoming darker in color. On light-colored 
varieties the spots are green, later becoming brown, while on red varieties 
the spots are at first red, then changing to black. The advancing margin 
is irregular and jagged and has a fringed appearance. 

3. That sometimes the spots are sunken and may or may not 
show this fringed margin, or they may be raised; gradations between the 
fringed smooth spot and the umbonate spot (raised and strongly projecting 
in the center). Where the spots are numerous, they may coalesce, forming 
large lesions involving a major portion of the fruit. On some varieties at 
least, like the Ben Davis, the skin ultimately cracks. The crevices, 
although usually about a half an inch long, may girdle the fruit and extend 
to the core. Often these cracks intersect, forming a cross. 

4. That within a few days after the spots become visible, black 
pimples (the fruiting bodies of the pathogene) develop thereon. 

Make a series of sketches to bring out points observed in 1, 2, 3, and 4. 

On the bark. The disease affects the fruit-spurs, twigs and rapidly 
growing shoots, producing characteristic cankers. Larger limbs and trunks 
are not usually affected. On the miaterial provided, observe: — 

5. That on the fruit-spurs, the spots are at first purplish or 
blackish. The center turns brown with age, the margin remaining pur- 
plish. The resulting lesion is small but rather conspicuous, with a crack 
along the margin. 

6. That on water-sprouts and on other rapidly growing shoots, 
the cankers have much the same appearance as that just described, but 
are larger, sometimes measuring an inch or more in length and often 
girdling the stem. Longitudinal cracks appear not only along the edge 
but also through the cankered tissue, eventually giving the surface a rough 
appearance. This character is noticable on cankers two or three years 

77 



78 

old. The canker may extend at several points along the margin, producing 
a lobed or somewhat concentric appearance. Frequently the canker is 
limited permanently by a crack and the wound is calloused. 
Make drawings to show the points brought out in 5 and 6. 

On the foliage. Examine the material provided and observe: — 

7. That the spots on the leaves are at first irregular, light- 
brown, yellowish or whitish and small, measuring 1.5 millirneters or less 
in diameter. Their distribution over the surface of the leaf is without 
order. On account of their minuteness, several may appear on a leaf 
without attracting attention, and perhaps without great injury; in more 
severe cases several hundred spots frequently appear on a single leaf. 
Badly diseased leaves may fall prematurely. The leaf-petioles may be 
girdled and instead of the leaves falling, they may die, turn brown, and 
hang on the tree. 

DRAW to illustrate the symptoms of the disease on leaves, as observed. 
(See PI. Ind. Bur. Bui. 144, pi. I.) 

ETIOLOGY 

The pathogene causing apple blotch is known as Phyllosticta solitaria 
Ellis and Everhart. This name applies to the asexual stage, no sexual 
stage being known. It may be regarded as an ascomycete, even in the 
absence of a perithecial stage, but until the missing stage is found, it is 
temporarily grouped with the Fungi Imperfecti. 

Life-history. A number of important links in the life-history of 
this pathogene remain to be discovered. Enough, however, is known to 
account for its whereabouts throughout the year. 

The Primary Cycles are initiated rather late in the spring, usually 
a month or more after the petals fall. 

Pathogenesis. Twig-cankers are the chief sources of the inoculum. 
The fungus hibernates in these lesions as myceliiim which becomes active 
in the spring and which produces pycnidia along the advancing margin of 
the canker. Examine the twigs provided, observe: — 

8. The minute black pycnidia in the bark near the margin of 
the cankers, draw to show their appearance under a hand-lens. 

Crush a bit of the bark containing pycnidia and examine for pycno- 
spores. observe : — 

9. That they are ovoid-elliptical, hyaline, one-celled, draw. 

10. That they sometimes exhibit a gelatinous sheath, as shown 
in PI. Ind. Bur. Bui. 144, pi. Ill, fig. 2-5. copy to show this. These 
pycnospores are washed or carried to the young fruits, twigs and leaves. 

Germination of these pycnospores occurs in twelve to eighteen hours. 
Examine the germinating spores and observe : — 

11. Number and position of the germtubes; branching, septa- 
tion and contents, draw or copy PI. Ind. Bur. Bid. 144, pi. Ill, fig. 6-7. 

The mycelium developed from the germtiibes ramifies the tissues of the 
fruit in a stellate fashion as may be observed by a careful examination 
of the lesions. Pycnidia are soon developed by this mycelium beneath 
the epidermis. Study prepared slides showing the pycnidia in section. 
observe : — 

12. The position and relation of the mature pycnidium to the 
host-tissues. 



79 

13. The structure of pycnidial walls and conidia-bearing struc- 
tures. 

Make a detailed drawing of the pycnidium and adjacent host-cells. 

The pathogene produces only sterile pycnidia on the leaves. Apparently 
on these organs the cycle is never completed. The pycnospores, matured 
in primary lesions on twigs and fruits, serve to initiate secondary cycles 
on all susceptible host-organs. In the twig-lesions, the pathogene hiber- 
nates either as immature pycnidia(?) or as mycelium from which pycnidia 
are developed in the spring. Thus is the cycle completed without the 
appearance of a sexual spore-form. 

Saprogenesis. There is no marked tendency exhibited by this 
pathogene to grow and develop' saprophytically. It may feed to some 
extent on the dead tissues after leaves and fruit have fallen. The forma- 
tion of spore-producing pycnidia only during the fore part of the season, 
followed by the development (in the later secondary cycles at least) of 
pycnosclerotia, suggests that perithecia, similar to those known for Guig- 
nardia Bidwellii on grapes, may be produced during saprogenesis. They 
are as yet unrecorded. 

Secondary Cycles originate from the pycnospore-crop of the pri- 
mary infections. They are initiated only during the first half of the sum- 
mer, and probably do not often initiate other secondary cycles. They 
duplicate in all respects the primary cycles. 

REPORT 

1. Compare and contrast in parallel columns the two diseases 
of the apple: — hlotch and sooty blotch. 



LEAF^CAST OF CONIFERS 

Species of the genera Pinus, Abies, Picea and Larix frequently suffer 
from needle-diseases known in common by the general name leaf-cast or 
needle-cast. These diseases have been much more abundant in Europe 
than in this countr3^ Frequently they have been epiphytotic on certain 
hosts in Europe. Although all leaf -cast diseases of conifers are very similar, 
they have various specific characteristics. The one considered in this 
outline is common on yellow pine {Pinus ponderosa Douglas) in the 
northwestern part of the United States. 

SYMPTOMS 

General symptoms. The leaves of certain branches turn brown at 
any time during the growing-season. In young trees, the leaves of the tips 
of all of the branches may be attacked while in older trees only the lower 
branches on the windward side usually show diseased needles. Witches'- 
broonis of large size are produced on older trees. Often the brooms are 
heavy enough to cause them to hang pendant from a larger limb. The 
recent growth of affected young trees is much deformed, due to the forcing 
of growth back of the tips of the twigs which are killed. (See Jour. Agr. 
Res. 6 : 277-288, pi. XXXII.) 

On the needles. The needles may be infected at any time during the 
growing-season. They show variable symptoms according to the time 
of year at which infection takes place and the point on the leaf where infec- 
tion was initiated. In the material provided, note : — 

1. The brown tips of certain needles. This is the first evidence 
of the disease when the needle becomes infected near the tip. 

2. In some cases the entire needle may gradually become straw- 
yellow in color and then turn brown later in the season. 

3. The long rows of black, shiny fruit-bodies of the pathogene 
on the dead needles. On which surface of the needle do they occur, dorsal 
or ventral ? draw. 

On the twigs. The mycelium, grows from the affected needles into the 
twig and produces hypertrophy. Later the affected twigs die. The branch 
bearing affected twdgs becomes gnarled and enlarged. Adventitious buds 
are forced and a witches'-broom is developed. In the specimens provided, 

OBSERVE : — 

4. The hypertrophied twigs and branches. 

5. The character of the brooms produced on the older trees. 
(See Jour. Agr. Res. 6, pi. XXXII.) 

DRAW showing the effect on the woody parts of the tree. 

ETIOLOGY 

The leaf-cast of yellow pine is caused by Hypoderma deformans Weir. 
Plypoderma is a genus of the Ascomycetes in which apothecia with valve- 
like coverings are formed. These valves open, exposing the hymenium, 
when abundant moisture is present. They close again during dry periods. 

Life-history. The life-history of this pathogene has only recently been 
investigated by Wier. (Jour. Agr. Res. 6 : 277-288.) Other closely 
related species causing leaf-cast of other conifers have been studied for 
years and the life-histories of all these species are simiilar. Where pycnidial 

80 



81 

forms are known, but little attention is paid to them since it seems that the 
asexual spores play no rdle in the life-history. Some believe that the 
pycnospores are not even capable of germinating. 

The Primary Cycle is initiated by the ascospores which are forcibly 
ejected from the open apothecia on dead needles at any time during the 
growing-season v.'hen sufficient moisture is present. 

Pathogenesis. The apothecia are formed on dead needles of 
healthy twigs and witches '-brooms. Although the twigs are penetrated 
and killed by the mycelium, the apothecia are formed only on the needles. 
In the material provided, observe: — 

6. The longitudinal slit running the length of each apotheciimi. 

DRAW. 

7. That, in material which has been in a moist-chamber, the two 
halves forming the covering of the apothecium are pulled apart exposing 
the hymeniimi, draw. 

In prepared cross-sections of needles with apothecia, note : — 

8. That the apothecium is formed imder the epidermis in the 
mesophyl-tissu e . 

9. The black mycelial covering of the fruit-body. 

10. The asci and paraphyses arising from the base of the fruit- 
body. 

11. The ascospores; two-celled and much longer than broad. 
DRAW showing the structure of the apothecium- and contents. 

On the same needles with the apothecia are the smaller asexual fruit- 
bodies, — the pycnidia. In the material, note: — 

12. The resemblance between the apothecia and pycnidia. 
Crush one of the pycnidia in water, observe : — 

13. The elongated conidia. The conidia of the leaf -cast patho- 
genes have never been shown to cause infections. 

The ascospores are shot from the asci when sufficient moisture is present 
to open the valves covering the hymenium. If material is available, 
note : — 

14. The milky deposit on the glass slides suspended over the 
discharging apothecia. 

15. By examination with the microscope, that this milky 
deposit is due to the large ntmiber of ascospores ejected upon it. 

Saprogenesis. There is no distinct or obligate saprophitic exist- 
ence required in the life-history of this pathogene. Leaves infected in 
early spring are killed and while still attached to the tree produce mature 
apothecia in the late autumn, although usually they do not mature until 
the following spring or summer. Pycnidia are formed among the apothecia 
at the same time. 

Secondary Cycles are apparently lacking in the case of this pathogene, 
since it takes the fungus a year to develop new inoculum. 

report 
1. Formulate control measures for this disease in a nursery 
which is surrounded by a mixed forest containing yellow pine. Give 
reasons for all steps in proposed measures based on the facts known about 
the life-history of the pathogene. 



BLACK ROT CANKER OF APPLE 

This disease affects chiefly the apple, but other pomaceotis fruits are 
by no means free fron the trouble. Marked variation in susceptibility 
of varieties is shown only where the canker-form of the disease appears. 
In western New York the Twenty Ounce is noted for its susceptibility 
to the canker. As a fungous disease of the apple in this state, it is perhaps 
second only to apple scab, the chief losses occurring as a result of cankers 
on the large limbs. 

SYMPTOMS 

Lesions appear on limbs, leaves and fruit, known on each respectively, 
as cankers, leaf -spots and black rot. 

On the limbs. Larger limbs are more often affected than twigs or 
trunks. The lesions are cankers and are usually on the upper sides of the 
limbs. At first the bark is reddish brown in color and sunken. (See Cor- 
nell Bui. 379 : 62-63, pi. VIII, and IX, fig. 4). Study the specimens 
available and observe: — 

L The size of the lesion; its surface, appearance, shape, color 
and margin. Is the margin definite? 

2. The pimple-like fruit-bodies of the pathogene ; their ntimbers 
and distribution within the lesion-area. Supplement the above with a care- 
ftd study of the illustrations in Cornell Bui. 379 to show the characters of 
the cankers. 

On the leaves. The first evidence of a spot on the leaf is the appearance 
of a minute purplish speck which soon enlarges until it has reached a 
diameter of 2 to 10 millimeters. It then becomes brown. Examine the 
the specimens provided and observe: — 

3. That the spots are at first circular, but that older ones show 
a lobing or a concentric appearance due to secondary spreading ; the center 
appears lighter in color and the whole effect is that of a frog's e^^e. The 
disease is therefore called "frog-eye" by certain writers. 

4. The presence or absence of the fruit-bodies of the pathogene. 
If present they will be much smaller than those on the cankers. 

Study, in addition to the specimens, the illustrations in Cornell Bill. 379, 
pi. VII, fig. 3; Virginia Bui. 209; PI. Ind. Bur. Bui. 121, pi. Ill, fig. 1-2. 
Defoliation results in cases of .severe infection. (See last named bulletin, 
plate IV.) 

On the fruit. Black rot is primarily a rot of ripe fruit, although green 
fruits may be attacked. Symptoms may show an^ovhere on the surface. 

Study, the material and illustration specimen provided, observe : — 

5. That the lesions appear first as small brown spots, which may 
turn dark at once or remain brown until the entire fruit is involved. 

6. Concentric color-zones in the lesion. 

7. The firmness of the rotted tissues ; taste and odor. 

8. The black fruit-bodies of the pathogene just under the 
skin; their distribution and arrangement within the lesion. 

DRAW to show the characters brought out above. (See Cornell Bui. 
379, pi. VII, fig. 1-2.) 

The lesion usually surrounds an injury of some sort, as for example : — 

9. A codling-moth wound in either green or ripe fruits, as seen 
in the illustration specimens, draw. 

82 



83 

10. The blossom-end of the fruit, a type common in Missouri 
and other regions. 

DRAW from illustration specimens or from figures in the btilletins. 

After a month or so the entire fruit becomes a waxy and finally a dry 
mummy which may fall or may hang on the tree for a year or more. Study 
specimens provided and show in a drawing the character of these mummies. 

ETIOLOGY 

The name of the pathogene is Physalospora Cydoniae Amaud. Until 
recently, it was known as Sphaeropsis Malorum Berkley which name is 
most commonly found in literature. The pathogene is very closely related 
to Guignardia BidwelUi (Ellis) Viala and Ravaz, the cause of black rot 
of grapes. Both organisms have similar perithecial and pycnidial stages. 

Life-historj'. This pathogene is evidently a facultative parasite. 
It develops most vigorously as a saprophyte on dead plant-tissues. It 
depends almost entirely on its asexual spores for propagation. The 
perithecial stage appears but rarely, at least in northern United States. 

The Primary Cycle is initiated by pycnospores, or by the ascospores, 
when the latter are produced. Only the former will be considered here. 
Pathogenesis. The fungus passes the winter as pycnospores in 
pycnidia and as mycelium in the cankers from which pycnidia with pycno- 
spores are produced. Remove several of the pycnidia from the cankered 
bark provided and crush in a drop of water on a slide. Examine under the 
microscope and observe: — 

11. The great number of pycnospores; their color, size, shape 
and septation. draw to show variations in the spores. 

These pycnospores constitute the chief primary inoci.dum. Washed 
by rain or possibly carried by insects to leaves or to wounds in the fruit 
or to dead tissue in the bark of the limbs, these spores germinate and ini- 
tiate the first infections. Study the germinating spores provided and 
observe: — 

12. The long germtubes; color, contents, septation and branch- 
ing. How many from each spore ? draw. 

From the infection-court the mycelium penetrates in various directions 
into the living tissues, killing them as it spreads. 

To study the mycelium in the tissues, mount a bit of the rotted fruit 
well teased apart, or study prepared slides of leaf -lesions in surface view, 
or sections through diseased bark, observe: — 

13. The color, size, septation and manner of branching. How 
does this mycelium compare with the germtubes from the pycnospores? 

14. The relation of the mycehum to the host-cells (best seen in 
rotted fruit-tissues); inter- or intracellular? Are haustoria developed? 
(See Cornell Bui. 379, fig. 31.) 

draw to illustrate the character of the mycelitun in the tissues. 

The mycelium near the center of the lesion soon begins to form gnarls 
of hyphae which rapidly develop into pycnidia containing pycnospores. 
(See Cornell Bui. 379, fig. 23.) Study the sections (freehand or prepared) 
through pycnidia. observe: — 

15. The dark globose structures, seen in longisection ; their 
relation to the tissues in which embedded; the ostiola. 

16. The structure of the pycnidial walls; origin and shape of the 
conidiophores; the spores and manner in which formed on the conidio- 



84 

phores. When ripe these pycnospores are disseminated and initiate 
secondary cycles. 

Make a drawing of a section through the pycnidium. (See Cornell Bui. 
379, fig. 22.) 

Saprogenesis. The fungus readily infests the dead bark of a 
great many plants. (See list in Cornell Bui. 379: 95 and 98.) Pycnidia 
may be formed on such plants. Occasionally perithecia are also formed 
from the saprogenic mycelium. The conditions under which they are 
produced are not clearly understood. 

Study sections through a peritheciiun under the demonstration micro- 
scope. Make a drawing to show its structure. (Compare Cornell Bui. 
379, fig. 18.) 

Secondary Cycles are not different from the primary except as to 
the time of year at which they occur. 

Pathological Histology. The lesions produced on the host are necrotic. 
Laboratory studies of histological changes will be confined to the more 
striking features of the canker. (Read Cornell Bui. 379:111-116, and 
study figures 34-37.) 

The myceliimi attacks the cortex, phloem, cambium, medullary rays, 
vascular bundles and pith. The outward changes evident to the naked 
eye, however, are the results of a necrosis of the cortex and phloem. These 
tissues are affected most generally and most extensively. Study slides 
showing sections cut through the center of limb-cankers and make : — 

17. Diagrammatic drawings of both cross- and longisections 
through the center of the canker, showing healthy tissue above and below 
the lesion. 

The fungus breaks away from the lesion at different points and spreads 
in a streak up and down the limb. If this is attempted in the cortex, 
the advance of the fungus is halted by the development of a cork-layer. 
There results, then, a pocket. If the pathogene leaves the cortex and 
advances in the sap-wood, no check on its progress is made by a cork- 
layer as just described above. On the other hand it enters the sap-tubes 
of the xylem and travels a considerable distance above and below the canker. 
As it does so the wood-cells, adjacent to the xylem-ducts, are discolored 
and filled with a brownish deposit. Consequently a streak is visible to 
the unaided eye where the mycelium is advancing, show this streak in 
the diagram of a longitudinal section through the canker. 

Study prepared sections through the margin of a young canker. 
NOTE : — 

18. The appearance of the tissues involved. Study the margin 
between healthy and diseased tissues. What sort of tissue is evident? 
Find the mycelium. Make a drawing including a portion of healthy and 
diseased tissues and the mycelitmi. 

Study prepared sections cut through the stem at some distance above 
the canker, draw to show points observed. 

REPORT 

1. Discuss the parasitism of Physalospora Cydoniae Amaud. 



ENDOTMIA CANKER OF CHESTNUT 

This disease has become the most destructive epiphytotic of tree 
diseases in this country. It is interesting in many respects. In the 
first place, the causal fungus is an importation, and the destruction caused 
is typical of what may often occur when nev/ and unsuspected pathogenes 
are transferred to a different flora and climate. Secondly, it is as good an 
example of efficient correlation of parasite with its host and of extreme 
susceptibility of the host, as any disease could demonstrate. 

SYMPTOMS 

The lesions of this disease appear chiefly in the body and larger limbs 
of the tree. The symptoms are of two general sorts : the lesions or cankers 
themselves, and the secondary effects produced by these cankers in un- 
invaded parts of the host. For convenience the latter are to be studied 
first. 

General symptoms. The diseased tree may often be detected at a 
distance by the following symptoms which are to be studied in the photo- 
graphs provided, observe : — 

1. That limbs with developing cankers, which have almost 
girdled them, do not produce normal leaves in the spring. They are 
usually lighter green in color and never develop to full size. This symptom 
is quite noticeable during May and June. (See illustration photographs 
2 and 3.) 

2. That dead leaves and last year's burrs often hang to limbs 
which have been killed by the fungus during the summer. (See illustra- 
tion photographs 3 and 4.) 

3. The dead limbs, the death of which can be traced to distinct 
cankers, that are of the type described below. (See illustration photo- 
graphs 1, 2, 3 and 4.) 

4. The abnormal development of suckers on trunk or limbs. 
Examination may prove that these suckers grow from below a cankered 
area. (See illustration photographs 2 and 3.) 

Cankers. The normal healthy bark of the chestnut remains smooth 
and green for several years and thus the cankers are very distinct on young 
growth. In the material and photographs provided, note: — 

5. That the cankers may be either sunken (necrotic) or raised 
and swollen (hypertrophic). (See illustration photographs 5 and 6.) 

6. That the surface of the diseased bark is always of a reddish 
cast, thus contrasting sharply with the green bark. (See frontispiece in 
Cornell Bui. 347.) 

7. That in a canker of the necrotic type, the bark is roughened 
and often split longitudinally; also the "pimples" or small eruptions all 
over the surface of the diseased area, — the fruiting stromata of the fungus. 
(See illustration photograph 5.) dr.\w. 

8. That in a canker of the hypertrophied type, the outer bark 
is split longitudinally because of the abnormal development of the tissues 
beneath. (See illustration photograph 6.) draw. 

'& The sunken cankered area, reddish brown in color, is the usual evidence 
of the disease. When the fungus attacks rough-barked portions of the 
tree, its effects are not noticeable externally until the stromata of the 

85 



86 

fungus break through in the cracks of the bark. This is the condition 
existing at the base of most trees attacked, and only the most careful 
scrutiny will reveal the lesion. (See illustration material.) 

ETIOLOGY 

This canker disease of the chestnut is caused by the pyrenomycetous 
fungus, Endothia parasitica (Murrill) Andersons. It is a member of the 
order Sphaeriales. The conidial stage, considered by itself, falls in the 
genus Endothiella of the Fungi Imperfecti. 

Life-history. Primary and secondary life-cycles are not as sharply 
differentiated in E. parasitica as is usual with pyrenomycetous pathogenes. 
This is due to the lack of marked correlation between the periodicity of 
spore-form production in the fungus and seasonal periodicit^^ Cycles 
are initiated by ascospores or conidia at any time when m.oisture and 
temperature permit. Those initiated early in the spring are to be regarded 
as the prim.ary cycles. 

The Primary Cycles may be initiated, as indicated above, by either of 
two kinds of inocula. 
Pathogenesis. 

(a) Ascospores are produced within perithecia developed in 
superficial stromata on the older portions of a canker or on dead twigs 
and bark. The production of ascospores is not limited to any specific 
time of year. It takes a certain time (depending on seasonal conditions) 
to develop perithecia after a new infection. If this period is interrupted 
by winter, development is simply arrested until spring. Therefore only 
those perithecia which were in a certain stage of development are mature 
in the early spring. 

In the material and photographs, note : — 

9. The perithecia! stromata, dark chocolate-brown in color. 
The surfaces of the mature stromata are covered with small projections or 
papillae, each with a small black dot at the apex. This is the ostiolum 
of a perithecium embedded in the stroma. (Illustration photograph 11.) 

DRAW. 

Crush a bit of this stroma and observe : — 

10. The ascospores; two-celled, hyaline, eight in each ascus 
(photographs 34-37). draw. These ascospores are forcibly discharged 
by the bursting of the asci as they accumulate on top of the papillae after 
being crowded out through the ostiolmn. 

If material is available study the violent discharge of the spores. With 
the hand-lens, observe: — 

11. The tiny points of light popping from the ostiola of the 
perithecia. 

Make a sketch to show ascospore-discharge. 

Hold clean dry slides at different heights above this material and 
determine the distance to which spores are shot in a vertical direction; 
also in a horizontal direction. From these slides, note: — 

12. That the whole contents of the ascus is ejected at once. 
Ascospores germinate as soon as discharged, if they fall in water, draw 
a germinating ascospore showing the germtubes (Cornell Bui. 347:568). 

(b) Conidia are produced within subcortical or superficial 
pycnidia (illustration photograph 10a and photograph 12). Pycnidial 



87 

development may be arrested at any stage by winter conditions and be 
resumed in the spring. Study the material provided and observe: — - 

13. The long, twisted, yellow, tendril -like masses of pycno- 
spores as they are pushed out from the subcortical pycnidia which appear 
as "pimples" on the thin-barked twigs. Study either fresh material in 
moist-chambers or photographs. These strings of spores are composed 
of thousands of the pycnospores stuck together by a mucilaginous substance. 
(See illustration photographs 8, 9 and 10.) draw. 

Make a mount of a portion of one of these spore-threads in a drop of 
water on a slide and note: — 

14. That the spores in the yellow string separate immediately. 
Place under the microscope and observe : — 

15. The small hyaline pycnospores. They are among the smal- 
lest pycnospores of the fungi. (See illustration photograph 13, d and e, 
and photographs 13-14.) 

Some conception of the large number of pycnospores produced from one 
canker may be obtained by noting the number observed in the mount 
from only a portion of one small thread. These spores, on account of 
their clinging together, cannot be blown about by the wind. Then too, 
the fact that they separate when water is present, would indicate that 
eventually these conidial strings find lodgment very near the place of 
formation, if not carried av/ay by insects, birds or other animals. The 
possibility of bark-boring insects and insectivorous birds acting as in- 
oculating agents in transferring these conidia to a health}^ tree is plausible. 

These pycnospores will not germinate in pure water but require the 
presence of substances from the bark in solution. The process of gennina- 
tion begins with enormous swelling of the minute spore. Study pycno- 
spore-germination, either from slides or from Cornell Bulletin 347 : 566. 

draw to show three or more stages in spore-germination. 

Ascospores or conidia, when introduced into a wound of any sort in the 
bark which exposes the tissues beneath the cork, germinate and develop a 
mycelium. (See illustration photograph 12.) 

The further developments of the pathogene are the same regardless- 
of whether ascospores or conidia initiate the cycle. The period of incuba- 
tion is an uncertain one depending on many factors; weather conditions 
(especially temperature), depth of inoculation and thickness of bark. 
In general, however, in the thin bark of twigs, definite lesions may be 
discerned three weeks after .inoculations made with conidia, and four 
weeks after inoculation with ascospores. 

Wounded tissue and a brief saprophytic growth, sufficient to produce 
mycelial fans, are necessary before living tissues can be invaded and infec- 
tion accomplished. Examine material and illustration photograph pro- 
vided. OBSERVE : — 

16. The form, size and color of these mycelial fans; relation 
to the host -tissue. 

17. In mounts under the microscope, the structure of the fans, 
branching of hyphae and relation to each other. 

DRAW to show general appearance of the fans and also their structure. 

After infection is accomplished and the mycelial fans begin to invade 
and kill living bark-tissues, the canker develops rapidly. Pycnidia are 
produced abundantly just back of the advancing edge of the canker.. 
Study material or photographs of young cankers and note : — 



88 

18. The appearance of the "pimples", — the subcortical pycnidia. 
(See illustration photograph 8, and photographs 1-5.) 

Make sections or study prepared slides of sections through the pycnidia. 
NOTE : — 

19. The mass of myceliLim of the fungus aggregated in the outer 
bark just at the base of the fruiting structures. Such a "bed" of mycelitim 
is termed a stroma. (See illustration photograph 2c.) 

.20. That this stromatic layer of mycelitim encases the spore- 
bearing cavities. The fruit-bodies are said to be encased or enclosed in a 
stroma. 

21. The irregular convoluted cavities in the stroma, — the pycni- 
dia. (See illustration photograph 13c.) 

22. The compact layer of pycnospores lining the entire inner wall 
of the pycnidium (photograph 6). 

23. The manner of the cutting-off of the pycnospores (photo- 
graph 6). 

24. That each stroma contains a single pycnidium and that 
the spores produced are pushed oiit of the ostiolum through a rupture in 
the bark in the long thread noted above. These spores are disseminated 
and initiate secondary cycles. 

Make drawings to show these points. 

After a short period of pycnospore-production, the stroma grows to the 
extent that the bark is pushed back and a cushion-like structure is raised 
considerably above the surface. These stromata are at first reddish in 
color, but later at maturity of the spores they are a chocolate-brown and 
covered with nimierous small projections or papillae. (See illustration 
photograph 11 and photographs 23, 30-33.) In the material provided, 
OBSERVE : — 

25. The extent of the stroma as compared with that of the 
pycnidial stage. (See illustration photograph 13c.) 

26. The cavities here are simple flask-shaped perithecia. (See 
photograph 11 and illustration photograph 14.) 

27. The thick, lead-colored walls of the perithecia. 

28. The long black necks leading from the perithecial cavity 
to the surface of the stroma where they open through the papillae by the 
ostiola. (See illustration photograph 14.) 

29. The numerous asci contained in the perithecia and floating 
about (photographs 34-37). 

DRAW showing the perithecium in longisection. 

The ascospores produced on the canker are washed down the tree and 
can initiate secondary infections at any slight wound in the bark. Event- 
ually the tree becomes covered with cankers and dies. 

Saprogenesis. The fungus does not require saprogenic conditions 
for the continuance of its cycles. On the other hand it is capable of exist- 
ing as -a saprophyte producing both spore-stages, the perithecia following 
the pycnidia as in pathogenesis. Whenever a saprogenic habit is assumed, 
it is purely incidental and not a definite stage providing for something 
which cannot be produced during pathogenesis. Strictly speaking, 
however, the perithecial stage may, although produced abundantly on 
cankers on living trees, be considered as the saprogenic stage since it is 
developed far back from the advancing margin and probably is largely 
or entirely fed by saprophytic myceliiim. 



89 

In the Secondary Cycles all the structures and phenomena of the 
primary cycles are exactly repeated. Secondary cycles differ in no way 
from the primary except in the time of year at which they are initiated. 

Pathological Histology. The mycelium of the fungus kills the ceUs 
of the bark, cambium, and sap-wood. The ceUs are killed in advance 
of the mycelium, presumably by excreted toxins. 

In the sections of cankers provided, observe : — 

30. The discoloration of the tissues affected. Make a diagram- 
matic DRAWING illustrating the enlargement of the canker. 

31. The septate and abundant mycelium. Often mycelial fans 
are formed in the bark. By stripping off the bark of a fresh canker, 
this is easily observed. (See demonstration microscopes for cross-sections 
of healthy and diseased twigs or see Phytopath. 4:191-200.) 

REPORT 

1. Bearing in mind the pertinent facts of the life-history of E. 
parasitica (Murr.) Andersons, enumerate and discuss the reasons why the 
Endothia disease is difficult to control. 



RHIZOCTONIA STEM=EOT 

This disease affects a great variety of plants. It appears to have an 
extensive range, occurring both in the new and the old world, in the tem- 
perate zones and in the tropics. 

SYMPTOMS 

The symptoms vary considerably depending upon the host affected. 
Studies here will be confined to seedlings, potatoes and carnations. 

On seedlings. The disease on seedlings is often known as damping-off. 
The base of the stems are affected, sometimes also the roots. Examine 
the diseased seedlings provided and observe : — 

1. The form, size and color of the lesions. 

2. Their location on the stem with respect to the surface of the 
soil. 

3. The depth to which the lesion penetrates (cut across the stem 
through the lesion). Are any of the stems girdled? 

4. A rotting of the roots in some seedlings. 

5. In some cases, the reddish brown mycelial strands or mats of 
the pathogene in the lesions. 

The symptoms of this disease in seedlings may be those resulting 
from a stem-rot or root-rot. They vary slightly with the seedling-host 
affected. (See Illinois Bui. 189:308-337.) 

Make sketches to show the symptoms of the disease as exhibited 
in the seedlings provided. 

On potatoes. A striking field-symptom is a marked unevenness in 
the stand. Many hills are wanting or the shoots are small and weak. 
If on digging up the seed-tubers, where plants have failed to come up, 
diseased sprouts are found, it is an almost certain evidence of this disease. 
Examine the specimens of diseased sprouts provided and see Maine Bui. 
230, fig. 62-67. observe :— 

6. The dark-colored canker-like lesions; their size, location on 
the sprouts and depth to which they penetrate. 

7. That some of the sprouts may get above ground before they 
are rotted off. They may even develop weak plants which grow slowly 
and persist for a time. 

8. That many of the sprouts which are rotted off send up new 
sprouts from a node below the lesion. These may in time become 
diseased and even rot off or they may produce nearly normal tops. 

Make a series of sketches to show the various symptoms and effects 
exhibited by diseased potato sprouts. 

Study the older stalks of diseased plants provided, observe : — 

9. The necrotic lesions at the base of the stem just below the 
soil-level. In some specimens the stem is nearly rotted off. 

10. The effect of the disease as exhibited in the tops, especially 
the production of clusters of small tubers about the base of the stem at 
the stirface of the soil and the production of aerial tubers in the axils of the 
leaves. 

11. The rosette-svmptoms exhibited by the tops of some affected 
plants. (See Ohio Bui. 139. fig. 4; and 145, fig. 1.) 

90 



91 

Make drawings to show symptoms exhibited in the older stems and 
tops. 

A variety of symptoms have been described for the tubers of affected 
plants. In the specimens or photographs provided, observe: — 

12. The large number of small potatoes from a diseased hill as 
compared with those from a healthy hill. Lesions on the tuber-stolons 
cut the young potatoes off before they can mature. The plant is thus 
stimulated to a production of more tubers which in turn are likewise pre- 
vented from reaching mature size. (Maine Bui. 230, fig. 69.) draw. 

13. That tubers themselves maybe affected, showing a russeting 
or even deep cracking of the surface. Cankers or pits in the tubers are 
held by some investigators to be frequent symptoms of this disease. 
(Maine Bui. 230, fig. 72-73.) draw. 

The most striking and certain diagnostic sign of this disease is the 
presence on tubers of the sclerotia of the pathogene. Examine the tubers 
provided, observe: — 

14. The numerous, dark-brown or black sclerotia scattered over 
the surface of the tuber. 

15. Their irregular size and shape. 

16. That on attempting to scrape them off, they cling very 
tightly, but when they come away it is evident that they are entirely 
superficial, draw a tuber to show the character and distribution of the 
sclerotia. These sclerotia are not usually associated with lesions on the 
tuber and are exceedingly common on potato-tubers, even from apparently 
healthy plants. 

Sclerotia may appear on the stems or roots as well as on the tubers. 
(See illustration specimens.) 

On the carnation. This is probably the most serious carnation disease 
with which florists have to contend. Both cuttings and older plants 
are affected. Cuttings are attacked below the surface of the soil and rot 
or damp-off like seedlings. 

Examine the specimens of diseased plants provided, observe: — 

17. The silvery gray color of the foliage as compared with the 
grey-green of the healthy plants. 

18. The erect and withered habit of the diseased plants. 

19. The soft and rotted cortex of the stem just at, or above, the 
surface of the soil. It comes away very readily from the. wood. 

20. In the crevices of the bark, the dark-brown sclerotia of the 
pathogene; not always easily discovered. 

draw to bring out the contrast, so far as you can, between diseased and 
healthy plants. 

ETIOLOGY 

The cause of this disease, so cosmopolitan both as to hosts and range, 
is the basidiomycetous fungus, CorHciuni vagum Berkley and Curtis, 
long known in its mycelial form as the sterile fungus, Rhizoctonia Solani 
Kiihn. 

Life-history. This pathogene presents some unusual features in its 
life-history. True conidial bodies are wanting. Sexual spores, the 
basidiospores, are formed, but apparently with no regularity and only 
under certain very favorable conditions. Sclerotia of a very simple type 
are produced in great abundance and doubtless are the only structures 
which serve to carry the pathogene over from one season to the next. 



92 

The Primary Cycle is initiated in the spring. The source of inoculum 
is, without much doubt, the sclerotia on overwintered host-debris, in the 
soil, or on resting organs of the host such as tubers or roots. 

Pathogenesis. Exactly how the primary infections are initiated 
does not appear to have been obsen^ed. The following studies suggest 
the probable phenomena involved. Tease apart, in a drop of water on the 
slide, one of the sclerotia from the material provided (on potato tubers 
or in pure culture). Examine and observe: — 

21. The barrel-shaped cells in profusely branched chains which 
make up the sclerotium. 

22. The color of the walls and the granular protoplasmic con- 
tents of many of the cells. 

23. That these cells break away readily in groups of several each. 
Make drawings of several of these cells together. 

Many of these cells germinate like conidia. Study the slides provided 
showing germination, observe: — 

24. The different types of germination presented, polar and equa- 
torial, 

25. The diameter of the germtubes as compared with that of the 
parent-cell and the number of germtubes from each. 

26. The granular and vacuolate character of the contents of the 
germtubes. 

27. The difference in the thickness and color of cell-walls in 
germinating cells and germtubes. 

DRAW several germinating cells to show the different types observed. 

These barrel-shaped cells constitute the inoculum. Just how they find 
their way to the infection-courts is not always clear. The germtubes 
may be capable of growing for considerable distances through the soil 
to the susceptible stem or root which they penetrate and infect. 

The vegetative mycelium, developed from these germtubes, is quite 
different in character from that composing the sclerotia. Mount some 
of the growing mycelitun from the advancing margin of the agar culture 
provided, observe : — 

28. The long parallel hyphae, much branched and frequently 
anastomosing. 

29. The characteristic manner of branching: the acute angle 
formed by the branch; the construction of the branch at the point of 
origin; and the septtun laid across it a short distance above the point of 
origin. These characters usually hold for the actively growing mycelium 
in the tissues of the host and serve as a diagnostic sign of the disease. 

30. The colorless character of the young mycelium ; the yellow- 
ing and browning with age. 

Make thin longitudinal sections of diseased stems (or use prepared 
slides) and observe: — 

31. The mycelium in relation to the cells of the host-tissue; 
inter- or intracellular.'* What tissues are invaded? How does the my- 
celium here compare with that studied from the culture? 

DRAW a portion of the section in detail to show the character of the 
mycelium and its relation to the cells. 

Hyphae may grow out from a diseased plant and, spreading through the 
soil, reach and infect nearby healthy plants. It is probable that external 
mycelium and sclerotial cells, formed in the soil, may be carried in cultiva- 



93 

ting and in other ways to the neighborhood of healthy plants and so 
initiate secondary cycles. 

Saprogenesis. The evidence seems to show that this pathogene 
may vegetate as mycelium in old host-debris and often in the soil. At 
least it develops on dead plant-parts and many strains appear to be almost 
obligate saprophytes. 

A sexual form of the pathogene, the Corticium stage, is at times de- 
veloped, and always on the surface of the living plant-parts above the 
ground, i. e. on stems and leaves. Apparently it does not always occur in 
the life-cycle. It may appear on the healthy parts above a Rhizoctonia 
lesion or on the stems and leaves of plants which show no such lesions or 
other evidences of injury. There is no indication that the fungus is ever 
pathogenic in this stage. 

Examine the material provided, observe: — 

32. The thin, white, weft-like coating over the surface, very 
difficult to discern in dried or pressed material. 

Scrape and mount some of this coating; examine under the micro- 
scope and OBSERVE : — 

33. That it is made up of a much-branched and septate mycelium. 
How does it compare with the vegetative mycelium of the Rhizoctonia 
stage as to size, branching and septation? 

34. The swollen tips of many of the branches each bearing two 
to four pointed sterigmata. These are the basidia. 

35. Scattered through the mount, the small, hyaline, ovate 
basidiospores. Some half-formed or nearly matured spores still attached 
to the sterigmata may sometimes be found. 

Make a drawing of the mycelium of the sexual stage, showing the fruit- 
ing structures or copy from Colorado Bui. 91, pi. III. 

The mycelium of the sexual stage appears to develop from the sclerotia 
on the underground parts of the host, or from the mycelium in the dead 
tissue of lesions, or from host-debris in the soil. It spreads upward over 
the surface of the living parts, without causing any direct injury, to a 
position where spores may be formed and readily disseminated. 

The basidiospores are scattered by the wind. It is almost certain that 
they never function in directly infecting a healthy plant but germinate 
and develop a saprophytic mycelium which may go into the sclerotial 
stage or at once attack healthy plants. 

The development of a considerable amount of saprophytically nourished 
mycelium in and about the infection-court seems to be a necessary prelude 
to an attack on living tissue. 

Secondary Cycles are frequently initiated during the growing-season 
by spreading or broken-ofE, vegetative mycelium. Sclerotia are readily 
formed and their barrel-like cells germinate at any time when conditions 
favor, so that they are doubtless a frequent source of secondary inoculum. 
So far as known, secondary cycles duplicate the primary in all details. 

REPORT 

1. Entimerate the facts in regard to the morphological structures 
and life-habits of C. vagum B. & C. which make its control difficult and 
explain why in each case. 

2. Outline a plan for reducing the ravages of this disease in one 
of the crops which it affects. 



POLITORACEOUS WOOD-ROTS OF TREES 

The wood-rots of trees, as a class of diseases, cause more economic loss 
than any other t\-pe of enpmtotic disease of trees. The diffictdty in con- 
trolling wood-rot diseases is due largely to their insidious nature and to the 
relatively small amount of work which has been done on them. The 
control of wood-rots in the forest is one of the important measures in 
forest-management, which must soon be recognized. Since the fruit- 
bodies of the ftangi causing wood-rots are the chief signs by which these 
diseases may be recognized, it is important that the student gain a working- 
knowledge of their identification. 

SYMPTOMS 

Sections from different deciduous and coniferous trees, showing the 
character of a few common wood-rots, are pro\"ided. observe : — 

1. That the decayed wood is. in some cases, darker in color than 
in the normal, usually reddish or brownish. Such wood-rots are called 
"red-rots." The color is caused by residual substances left by the causal 

fungus. DRAW. 

2. In other cases the rotted wood is Hghter in color than the 
normal wood and often is almost white. Such wood-rots are called "white- 
rots." The change in color is due to the delignification of the wood-ele- 
ments, lea\-ing the whitish ceUtdose cell-walls. Xo colored residue is 
left as in the red-rots. draw. 

3. That certain wood-rots at first appear in the heart-wood 
and work out toward the sap-wood, while others work froin the start 
chiefly in the sap-wood. The former are known as "heart -rots" and the 
latter as "sap-rots." draw. 

4. That, in some wood-rots, the tissue is entirely decayed lea\'ing 
holes or pockets. Usually these holes are surrounded by a white border 
that represents the cellulose fibers which were not dissolved, draw. 

5. That, in other cases, the decay results in a uniform change to 
a weak cellulose structure but in no place are the tissues entirely destroyed. 

DR.\W. 

6. The black lines bordering the decayed portion of the wood in 
many wood-rots. This line is usually due to both the color of the feeding- 
h^-phae and to the first products of the decay-process in solution in the sap. 

DRAW. 

ETIOLOGY 

The majorits- of wood-rots are caused by basidiomycetous fungi be- 
longing to the two large families. Agaricaceae (toad-stools) and Poly- 
poraceae (poh-pores or bracket -fungi) . The poh-pores are by far the more 
important of the two groups. 

The main distinction between the Agaricaceae and Polyporaceae is in 
the morphology- of the under side of the fruit-body. The h}Tnenium, each 
basidiiim of which bears four sporidia on sterigmata. is spread over the 
plate-hke or tube-hke surfaces which hang vertically from the cap or pileus. 

Examine the prepared sections of the h^-menium of a toad-stool or 
pol_\-pore with the microscope, obsera'e : — 

7. The irregular tangled mycehum making up the trama or 
supporting-structure for the h}-menitun. 

94 



95 

8. The more or less regularly arranged tips of hyphae extending 
at right angles to the general direction of the trama-hyphae, — the sub- 
hymenium. The hyphae of the subhymenium always lie in a horizontal 
direction when the fruit-body is in place. 

9. The layer of basidia borne on the tips of the hyphae of the 
subhymenium. Each basidium has four sterigmata on the free end. 
The layer of basidia and the spores borne on them make up the hymenium. 

10. The four spores borne on each basidium, one on each 
sterigma. 

Make a drawing to show the structure of the trama, subhymeniiun, 
basidia and spores in relation to one another. 

Study the specimens of toad-stools and polypores on the tables and 
classify each nuinbered specimen as to the family to which it belongs, 
Uvsing the following key: — 

A. Hymenium usually on the underside in the form of radiating 

plates or strong folds Agaricaceae 

B. Hymenium usually below or on the outer surface when the plant 

is spread over the substratum; honey-combed, porous, tubulose 
or reticulate Polyporaceae 

The numbered specimens of polypores on the tables are next to be 
studied and the genus to which they belong determined for each. All the 
steps in using the key must be written on sheets provided, to show how 
each determination was arrived at, thus: — 1. Polyporus. 2, B, b, x,"**. 
The older terminology is used here because of its use in most of the litera- 
ture and because it is the more simple. The rearrangement of the mem- 
bers of this group and the new genera proposed may be consulted in North 
American Flora 7, and Mvirrill, Northern Polypores, p. 1-64. 

The following key to the Polyporaceae (i.e. the Polyporeae as given in 
the text on nature of decay in wood) is rearranged from that of Over- 
holt, The Polyporaceae of Ohio. Missouri Bot. Gard. Ann. 1 :81- 155. 

1 . Sporophore entirely resupinate ; pileus none Poria 

2. Sporophore sessile or stipitate, sometimes effused-reflexed but nor- 
mally not entirely resupinate. 

A. Hymenium either daedaloid, labyrinthiform or lamellate, at 

least in part. 

a. Context white. 

x. Pileus minutely velvety to glabrous; context more than 
1 mm. thick Daedalea 

y. Pileus hirsute to villous; context 1 mm. or less thick, 

"Hymenium lamellate, at least in part Lenzites 

""Hymeniimi daedaloid but never lamellate . Daedalea 

b. Context brown. 

X. Plants woody and perennial, more than one cm. thick; 

hymenium not at all lamellate Trametes 

y. Plants coriaceous or corky, less than 1 cm. thick; 

hymenium often lamellate Lenzites 

B. Hymenium poroid or sometimes broken up into teeth. 

a. Hymenium broken up into teeth. 

X. Tubes or teeth 5 mm. or more long Irpex 

y. Tubes or teeth less than 5 mm. long. 

''Hymenium labyrinthiform at first and remaining 
so at the margin Daedalea 



96 

°°Hymenmm never labyrinthiform. 

IxPileus less than 1 cm. broad; fungus mostly 

resupinate Irpex 

[x^xPileus more than 1 cm. broad; fungus not 

mostly resupinate Polyporus 

b. Hymenium poroid, not broken up into teeth. 
X. Pores large and hexagonal ; stipe present. 

"Stipe lateral, often rudimentary; pores usually radi- 
ating and longer in the radial direction . . Favolus 
°°Stipe usually central or sub-central; pores not 

radiating Polpyorus 

y. Pores small, and circular or angular; stipe present or 
absent. 
"Tubes in a single layer; plants annual. 

[jiTubes not in a distinct stratum but appearing 
to be sunken to different depths into the 

context Trametes 

[xixTubes forming a well marked stratum entirely 

distinct from the context Polyporus 

""Tubes in two or several layers; plants perennial. 
txHymenium bright yellowish brown; plants 
growing only on the wood of coniferous 

trees Trametes 

[A^jiHymenium whitish, flesh-colored, dull brown, 
etc., but not bright yellowish brown; plants 
growing on the wood of either coniferous or 

deciduous trees Fomes 

The specimens of polypores, labeled as to name, should next be studied. 
The names of each shoiild be written down, together with an original 
description of the specimen in the student's own words. These notes should 
be as full as possible, especially as to all' the striking differences. 

Life-history. There is great similarity in the life-histories of the poly- 
pores causing wood-rots. Wounds in which the wood is exposed are re- 
quired for infection. The basidiospores, carried by the wind, constitute 
the only kind of inoculum except by the direct growth of the vegetative 
mycelium from a diseased individual to a healthy one. 

Primary Cycles only, exist, since there is only one crop of basidio- 
phores formed annually. A single cycle may require several years to be 
completed. 

Pathogenesis. The spores formed on the basidia are forcibly 
thrown from their sterigmata and fall out from between adjacent projec- 
tions of the hymenial surface (pores or gills). 

Examine figures in Buller, Researches on Fungi, and make diagram- 
matic DRAWINGS illustrating the anatomy of a toad-stool and polypore 
in longitudinal section, showing the path taken by an ejected spore in each. 
The type of infection-court required differs with the species under 
consideration. In general, those fungi causing heart-rots require very 
different types of wounds for successfiil infection from those causing sap- 
rots. In any case the basidiospore, borne by the wind may find lodgment 
in a suitable infection-court or, if not, it may later be washed by water 
to a suitable place. The spores are relatively short-lived and proper 
conditions for germination mUvSt be provided in a short time if the spores 



97 

are to function. Under proper conditions, the spore germinates and a new 
mycelium develops in the wood. The steps in the decay-processes from 
the time of infection to the production of the mature fruiting body differs 
with the species of polypore concerned and somewhat also with the kind 
of tree affected. In the case of the fungi which attack the woody parts of 
trees, delignification of the cell-walls usually takes place, at least the lignin- 
cellulose composition of the cell-wall is destroyed. It is supposed that 
an enzyme similar to hadromase, isolated by Czapek, is excreted by all 
wood-rotting fungi. The products of the activities of cytolytic enzymes 
then, form the main food for the fungus. Many other compounds, such as 
tanorins and proteids, are used by some of the wood-rotting fungi. 

Saprogenesis. The majority of the polypores may live purely as 
saprophytes. Some do so more commonly than others. However, 
this saprophytic habit is onl}- incidental, since it does not form a required 
phase in the life-history which produces anything that is not produced in 
the pathogenic phase. The same kind of inoculum, however, is formed in 
saprogenesis as in pathogenesis and this may initiate new primary cycles. 
Pathological Histology. To illustrate the process by which the my- 
celium of wood-rotting polypores causes the deca^^ of the wood, prepared 
cross-sections and longitudinal sections of beech wood, rotted by Fomes 
pinicola (Swartz) Fr., are provided. The sections are stained to show; 
lignified cell-walls, light green (miethyl green); delignified cell-walls or 
cellulose, brownish red (congo red) ; and middle lamella, light red (ruthen- 
ium red), observe: — 

11. The abundant and light-colored hyphae of the fimgus in the 
partially decayed wood. draw. 

12. The large dark-colored hyphae where new tissue is being 
delignified. dravv'. 

13. The delignification of the wood-elements. Which layer of 
the cell-wall is delignified first? 

14. The characters of sound beech wood, elements present, 
and thickness of walls of different elements. 

Make a drawing of healthy beech wood at a point shoAving both 
spring and summer wood with all the elements properly arranged and 
detailed. Label fully. 

15. The characters of the decayed wood, noting the changes 
brought about in the elem.ents. 

Make a drawing of decayed beech wood showing the same elements 
as in the drawing of healthy wood. Label fully, and bring out all the 
changes caused during the decay-process. If necessary, make drawings 
of the intermediate steps between the healthy and decayed wood, vising 
the cell-wall of a single element to illustrate the point. 

REPORT 

1. Hand in report-sheets filled out giving your determination 
of the family and genus of each of the numbered specimens. 

2. Hand in the notes taken on the named specimens and an 
original dichotomous key (arranged like the one in the outline) by which 
each species can be determined. 



MNG^-SHAKE OF SPRUCE 

This wood-rot of spruce is also common in pine, larch, fir and other 
conifers. It is not known to affect junipers. It has been estimated that 
this disease causes more loss than the aggregate of all the other numerous 
wood-rots. 

SYMPTOMS 

Trees affected by this disease, as in the case of other wood-rots, do 
not show any symptoms externally until limbs begin to die. The fruiting 
bodies of the causal fungus, produced at the broken stubs of limbs or from 
the fissures in the bark, are usually the first signs of disease. 

External signs. vStudy U. S. Agr. Dept., Veg. Phys. and Path. Div. Bui. 
25, pi. XII and the specimens provided, observe : — 

1. The various forms of sporophores which constitute the signs 
of this wood-rot. 

2. The sporophores may be shelving or more or less resupinate. 

3. The sporophores may emerge from broken stubs of branches 
or from the fissures in the bark. 

Make outline drawings from the specimens or plate referred to, showing 
the above points. 

Internal S3Tiiptoms. In the specimens of transversely and longitudinally- 
sawed wood of spruce affected by this wood-rot; and in plates VI, VII, 
and VIII of the above mentioned bulletin, note: — 

4. That the first effect on the wood is the red-brown discolora- 
tion. Such timber is said to be "red-hearted." 

5. That later in the decay-process the color becomes lighter. 

6. That white areas appear here and there in the wood in some- 
what later stages and that these areas gradually increase in number and 

size. DRAW. 

7. That the centers of the white areas are entirely dissolved, 
leaving holes surrounded by white borders, draw. 

8. Around most of the holes, a little distance away and usually 
midway between adjoining pockets, a definite black line. draw. 

9. That the size of the pockets and the amount of relatively 
sound wood left between the pockets depends on the kind of wood attacked. 

10. That the rotted area enlarges longitudinally in the wood 
much faster than transversely and often the decay involves either partially 
or completely a certain number of annual rings. This is the character 
which gives the name "ring-shake" to this wood-rot. 

ETIOLOGY 

The cause of wood-rot known as ring-shake of conifers is the poly- 
poraceous fungus, Trametes Pint Fries {=Porodaedalea Pint (Thore) 
Murrill). 

Life-history. The species of polypores which cause wood-rots of living 
trees are similar in all the major facts of their life-histories. (See pages 
96-97). 

The primary cycles are initiated by the basidiospores which are 
wind-borne. 

Pathogenesis . It has been shown that the basidiospores are shot 
from the sterigmata with just enough force in each case to bring them to 

98 



99 

the center of the perpendicular pore from which they fall and are then 
caught by the wind and blown about in all directions. (Btdler, Researches 
on Fungi, p. 133-152). If material of discharging spores is available, 
NOTE : — 

11. The clouds of spores falling from the sporophore. draw. 

12. The deposit of white spores on the black paper under the 
sporophores. 

Mount some of these spores in water and under the microscope, 
OBSERVE : — 

13. The size and shape of the spores, draw. 

The sources of the primary inoculum are the shelving or resupinate 
sporophores produced in great abundance on the outside of diseased trees. 
In the material provided, note : — 

14. That the fruit-bodies are perennial. 

15. The black, checked upper surface of the pileus in the case 
of the shelving form. 

16. The dull-brown pore-layer, — the compound hymenium. 

17. The small round pores making up the pore-layer. 
DRAW a shelving and resupinate fruit-body. 

Make freehand sections of a fruit-body provided, stain with eosin, 
mount and examine with the microscope, observe: — 

18. The character of the mycelium forming the trama of the 
pileus and the substance between the pores, draw. 

19. The subhymenium and the two kinds of cells produced 
in the hymenium, basidia and setae, draw. 

20. The form of the basidium; the four spores borne on each. 

DRAW. 

If material does not show points mentioned above, see Veg. Phys. 
and Path. Div. Bui. 25, pi. IX, fig. 3. copy. 

If the spores reach the exposed heart -wood of susceptible conifers, 
they germinate under favorable conditions and the mycelium extends 
down into the trunk of the branch. Why are exposed wounds in the sap- 
wood of conifers not infected ? 

The mycelium usually confines its activities to the annual rings first 
invaded, extending longitudinally and circumferentially in these rings 
much more rapidly than radially in the wood. Thus, completely decayed 
rings in the wood often result as the first evidence of disease. This is the 
ring-shake symptom. Soon, however, the mycelium invades the rings 
of wood adjacent to the shake and spreads inward and outward toward 
the sap-wood. The progress outward depends upon the amoiuit of resin 
present (i.e. in pine). The mycelium rarely penetrates the sap-wood. 
In spruce, however, it not only decays the sap-wood but penetrates and 
kills the tissues of the bark and produces small sporophores in the fissures 
of the bark. 

Saprogenesis. The ring-shake fungus may live in the wood of 
susceptible conifers after the tree is dead. Fruiting bodies are formed 
also in the saprophytic stage and furnish inoculum for new infections. 
Saprogenesis is not an obligatory stage, however, for the production of 
anything in the Hfe-history of the fungus which is not produced in the 
living tree. 

Secondary Cycles are not considered as existing in cases of wood-rot path- 
ogenes since only one source of inoculum is developed annually. The period 



100 

over which the dissemination of priman* inoculiun takes place may, 
however, be long, as the sporophores of polypores often cease spore- 
production under adverse weather-conditions. With more favorable 
conditions it is resumed. 

Pathological Histology. The decay-process caused by this fungus in 
spruce is of two kinds. In the cross- and longisections of spruce wood 
where pockets are not formed, note : — 

21. That the medullary ray-cells are destroyed and from these 
the mycelium spreads to the adjoining tracheids. draw. 

22. That the elements of the spring wood are destroyed before 
the thicker-walled summer elements. 

23. That the first noticeable change in the cell-walls is the separa- 
tion of the middle wall-layer from the outer layer. It has not been deter- 
mined that this is due to the shrinking of the middle layer as a result of 
the disintegration of the outer layer, draw. 

24. That when a hypha makes a hole through a ceU-wall the 
hole is at first cylindrical but changes by enlarging in the middle, draw. 

25. That the outer waU-layer of each cell separates from the 
outer wall-layer of the neighboring cell. draw. 

26. That, following this stage, the middle wall-layer is changed 
into cellulose. Then there are left for a time only the outer and inner 
lignified layers which later also change to cellulose, draw. 

27. That the middle layer disappears first, then the outermost 
layer is dissolved, draav. 

2S. That the onl}- portion 'of the wall resisting decay is the inner- 
most layer of the cell-wall. draw. 

In cross- and longisections of spruce wood where pockets are formed, 
NOTE : — 

29. The general appearance of the pockets under low-power; 
their location, abundance of m3xelium, and the black lines surrounding 
the pockets, draw. 

30. The black line tmder the high-power. Of what is it com- 
posed? DR.\W. 

31. The fine mycelium around the margin of the pocket. 
draw. 

32. That the progress of the decay around the margin of the 
pocket proceeds as follows: — The entire wall is changed into cellulose; 
the elements become separated by the dissolving of the middle-lamella; 
the outennost wall-layer then dissolves and the wall becomes thinner 
and thinner until only the innermost layer is left, draw showing the 
progressive stages. 

REPORT 

1. From the studies made of this decay-process, write a concise 
accotmt of the steps involved. (See U. S. Agr. Dept., Veg. Phvs. and Path. 
Div. Bui. 25:31-40, pi. VI-X.) 



IGNIAMUS HEART^ROT 

This is the most common of the num.erous heart -rot diseases of decidu- 
ous trees and may be taken as a type. The ftingi causing heart-rots of 
deciduous trees are more or less ahke in their hfe-habits and the methods 
for their control are similar. (In this connection, see von Schrenk and 
Spaulding, Diseases of Deciduous Forest Trees. PI. Ind. Bur. Bui. 1-49: 
1-85, pi. 1-10, fig. I-X, and extensive bibliography.) 

SYMPTOMS 

External signs. In the material provided, observe : — 

1 . That the only external evidence of the disease is the bracket- 
like sporophore of the fungus. 

2. The point at which the sporophore emerges. Why? (See 
PI. Ind. Bur. Bui. 149, pi. I.) 

3. The variation in the size of the sporophores. Upon what two 
factors does this variation depend '1 

4. The age of different sporophores. 

5. The characteristics, shape and color of the sporophores. Com- 
pare with other specimens on the demonstration table. 

Make a drawing to show characteristics of the sporophore in position 
on the host. 

Internal symptoms. In the transverse and longitudinally sawed 
sections of diseased wood provided, observe: — 

6. The rotted wood at the center of the lesion; white or yellow- 
ish white and more or less soft and crumbly. 

7. The distinct black line or lines bounding the white rotted 
areas. 

8. The Hght-brown or yellowish brown zone of firm hard wood 
just outside the outermost black line. Cause of the discoloration? 

9. The brown felty character of the mycelial growth from the 
cut ends of the sticks. 

Make drawings to show the symptoms as seen in the transverse and 
longitudinal sections. 

ETIOLOGY 

This disease is caused by a basidiomycetous fungus, Fmnes igniarius 
(Linneus) Fries, one of the perennial species of the family Polyporaceae. 

Life-history. This fungus, although largely confined in its activities 
to the heart -wood of its host, may be regarded as a true pathogene, be- 
cause if develops only in living trees. 

Primary Cycles only are known, the infections occmring once a year. 
With our present limited knowledge of this fungus, no sharp line can be 
drawn, if any exists, between its saprogenic and pathogenic activities. 

The source of inoculum is the compound sporophore on the li^dng tree. 
Study several of the sporophores provided and observe: — 

10. The cracked and checked, hard black upper surface of the 
sporophores. 

11. The uproUed reddish brown m.argin of the last -formed annual 
pore-layer. 

12. The brown surface of the pore-layer. 

101 



102 

13. The minute almost microscopic pores making up the pore- 
layer. Study with the hand-lens. The basidiospores sift forth from these 
pores, and are often discharged in such quantities as to be visible to the 
naked eye, forming a white coating on the bark and leaves beneath the 
sporophore. 

Mount some of the spores (discharged or in crushed mounts of the 
the pores). Examine and observe: — 

14. Their form, size and color, draw. 

Some of these spores, carried by the wind, find lodgment in dead 
branches or the stubs of broken-off limbs of their favorite hosts. Here 
they germinate sending forth a germtube into the dead host-tissue. This 
germtube develops into a mycelium which penetrates to the heart-wood 
where it spreads in all directions iDut most rapidly up and down, feeding 
and accumulating a reserve of food. 

The character of this mycelitun and its relation to the host -tissue can 
be best studied in prepared sections made through the region of the black 
line. Study the sections provided and observe : — 

15. The black line. Of what is it composed? 

16. The invasion of the medullary rays and w^ood-fibers sur- 
rounding the tracheae in advance of the black line. 

17. The character of the mycelium in the region of the black 
line and when advancing into uninvaded tissue. 

18. The character of the mycelium inside of the black line where 
the tissues have already been delignified. 

Make drawings showing the above points. 

After a year or more, a sporophore develops from this m^'-celium, 
usually emerging from the old infection-court. The lower surface devel- 
ops a pore-layer which is annually renewed thereafter for many ^''ears. 

Examine the sporophore that has been sawed through and note: — 

19. That it is composed of many pore-layers or strata, one below 
the other, each succeeding one being more extensive than the preceding. 
Normally one layer is formed each year. draw. 

20. The rusty brown color of the inside of the sporophore with 
white flecks through it. 

21. The length of the pores, that is, the width of the pore-layer. 
Are the pores continuous from year to year? 

22. The shape of the pore-openings or mouths. Compare 
them, as to size and vShape, with the pores of other species on the demonstra- 
tion table. 

Make a drawing of the longisection through the sporophore; also an 
enlarged drawing to show shape and arrangement of the pore-openings. 
Make thin cross-sections of bits of the pore-layer and note: — 

23. The mycelial structure of the pore-layer. 

24. The basidia with spores lining the inside of the pores. 
Detail in a drawing the structures exhibited in the cross-section of a 

pore. 

The spores sift out of these pores and are disseminated by the wind. 
Make a diagrammatic drawing showing the pores in longitudinal section 
and the path of a spore in getting out of the tube. (See Buller's book, 
Researches on Ftmgi, p. 189.) 

Pathological Histology. Prepared cross- and longisections of wood 
from a living maple tree are provided. These have been stained to show: 



103 

lignified cell-walls, light-green (methyl green); delignified cell-walls or 
ceUiilose, brownish red (congo red) ;. and middle lamella, light-red (ruthe- 
nium red). The dark color of the large vigorous hyphae when advancing 
into healthy tissue is natural. In the cross-sections, observe : — 

25. The dense grovi^h of the large dark-colored hyphae making 
up the black line. At this point the fungus is advancing in a solid line 
toward the sap-wood, drav; a few cells included in the black line, detail- 
ing the hyphae. 

26. The scattered areas, especially around tracheae and in the 
medullary rays, where the same dark-colored hyphae are found. How do 
these centers of decay originate ahead of the general advance? Cor- 
roborate your answer by two drawings; one diagrammatic, showing 
the relation between the black line and the advanced centers of decay; 
the other detailed, showing the actual cells. 

27. The accumulation of green-staining material around the cells 
newly invaded. What does this indicate? 

28. The delignification of the ceU-walls near the advanced 
centers of decay. Do the hyphae of the fungus have to be in contact 
with a cell -wall to cause its delignification? Proof? How is the delignifi- 
cation brought about? Detail answers in drawings. 

29. The character of sound maple wood; elements present and 
thickness of the walls of different elements. Make a drawing of healthy 
maple wood showing an annual ring with all elements properly arranged 
and detailed. Label fully. 

30. The characters of the decayed wood some distance inside 
the black line, noting changes in the elements. Make a drawing of the 
decayed maple wood showing the same elements as in healthy wood. 
Label fully and bring out all changes brought about by the decay-process. 

Study the longitudinal sections and observe : — 

31. The mycelium advancing into healthy tissue. 

32. The change brought about by the decay-process inside the 
black line as compared with the healthy tissue outside. Make drawings 
of healthy and decayed maple wood, contrasting the conditions as seen in 
the longitudinal sections of each. 

report 

1. Compare and contrast the decay-process caused by F. 
igniarius (L.) Fr. in maple with that caused by T. Pint Fr. in spruce. 



FENCE Pp§T=.ROT 

One of the items of expense on the farm is the building and keeping- up 
of fences. The Hfe of the posts is considerably shortened by various 
saprophytic ftmgi which often attack and quickly rot them. A knowledge 
of the character of these rots and of the conditions favorable to them 
is of ser\'ice in devising means of treatm.ent which greatly lengthen the life 
of the timber. 

SYMPTOMS 

Sections of affected timber are provided, observe: — 

1. The light-colored region about the margin, inside of the bark, 
— the affected wood. Compare the hardness of this with that of healthy 
wood. Is the bark similarly affected? 

2. That the affected wood in the early stages includes only 
the outer three or fotir annual rings, but that ultimately the penetration 
is deeper, showing that although the sap-wood is first invaded the heart- 
wood will finally also be destroyed. Compare with the blocks that have 
been affected longest. 

Make drawings of the transverse and longitudinal aspects. 

3. On the larger blocks, the shelving or resupinate sporophores 
of the fungus causing this decay. Why are some shelving and some 
resupinate? Make an outline drawing showing these sporophores 
attached to the post. 

ETIOLOGY 

The cause of this rot is a very common polyporaceous fungus, Polystic- 
tus versicolor (Linneus) Fries. The fungus is ordinarily a saprogene 
but is known to cause destructive heart-rot of catalpa. It attacks a great 
variety of timbers of broad-leaved trees but never affects coniferous wood. 

Life-history. There are apparently no secondary cycles, the infection 
occuring in the earlier part of the season with usually no prodtiction of 
fruit-bodies until the end of the season. There is of cotuse no pathogenic 
phase. 

The Life Cycles are initiated by basidiospores from the fruit-bodies 
during the wet weather of spring and early summer, and possibly also to 
some extent from early-matured sporophores in the autumn. 

Saprogenesis. Exaniine the black paper vmder the sporophores 
which have been in a moist-chamber for several hours, observe: — 

4. The tiny piles of white spores, each representing the spores 
discharged from a single pore. 

Mount some of these spores in water and examine under high-power. 
observe : — 

5. Their form, size, color and contents, draw. Study and 
DRAW to show spore-germ. inati on as seen in slides provided. 

The germtube penetrates to the dead sap-wood, where it develops 
a vigorous mycelium. This mycelium- spreads through the wood, dis- 
integrates it by means of enzymes and feeds upon the products of this 
disintegration. 
, Study a thin longisection of the diseased wood and observe : — 

6. The mycelial threads, especially abtmdant in the pitted vessels 
and medullary rays. 

104 



105 

7. The form, size, branching and septation of the mycelitim. 

8. The relation of the hyphae to the cells and vessels. Do they 
penetrate between or through the tissue -elements!" 

DRAW showing a few cells and vessels with the ramifying mycelium. 

This vegetative activity continues during the season. No conidia 
are formed in nature so far as known. Toward autumn the mycelium 
begins to form sporophores which break forth through the lenticels or 
cracks in the bark, as may be seen in the specimens provided. Examine 
the sporophores further and observe : — 

9. The velvety upper surface. Note the zonations and coloring. 

10. The lower or pore-surface. Note the numerous small pores; 
their form and size; color of the pore-surface. 

Make a drawing of a sporophore. 

Make thin cross-sections of the pileus and examine with high-power. 
NOTE : — 

11. That it is made up of interlacing strands of mycelium. 

12. The parallel columns which are the sides of the tiibes; 
lining these tubes the basidia bearing the spores. 

Make an outline drawing of the cross-section of a piletts to indicate the 
relation of parts. 

Carefully examine the hymenium and make out: — 

13. The club-shaped basidia, bearing at their tips four spores 
on short sterigmata. 

Make a large drawing of the hymenium showing its structure. 

These sporophores are not killed even by long drying and when wetted 
again mature and discharge more spores. A single mature sporophore 
will, if kept moist, continue to discharge spores for more than two weeks. 

The mycelium lives over in the rotten tissues and, spreading into 
adjoining sound wood, continues the destruction and produces a crop of 
sporophores year after year until the post is destroyed. 

Pathological Histology. In order to more clearly understand the 
changes which have taken place in the rotted wood, a study of the nonnal 
wood must first be made. 

Cut thin cross- and longisections of the healthy wood from the center 
of the blocks. Stain two minutes with methyl green and 45 seconds with 
Congo red. Examine carefully, observe: — 

14. The large tracheae in the spring wood. Note the heavy 
walls of these ducts; within the tracheae, the cell-like structures. These 
are ingrowths of the walls of surrounding cells, now dead, and are termed 
tyloses. 

15. The heavy-walled wood-cells suiTounding the ducts. 

1 6. The medullary rays between the xylem-wedges. The methyl 
green stains the lignified tissues ; and the congo red, the tissues the walls 
of which are partially or wholly of a cellulose nature. 

Locate each of these in the cross- and longisections. Make a detailed 
drawing of a part of the cross-section to show the relation of the tissues, 
the size of the cells and the thickness of the walls. 

Now examine the similarly stained cross- and longisections of the 
decayed wood, observe : — 

17. The dense masses of mycelium in the tracheae and the 
absence of the tyloses. Compare the walls of the tracheae here with 
those in the sound wood. 



106 

18. The mycelium in the wood-cells about the ducts and the 
medullary ray-cells. Examine the cell-walls. They are thinner than in 
the healthy wood. Note that the spring wood of each annual ring is 
more affected than the autumn wood. 

19. That some of the tissues in the diseased wood take the stains 
differently than they did in healthy wood ; which tissues ? In the diseased 
wood, the mycelium also is stained red. 

The destruction or removal of the lignin from the cell-walls explains 
the lighter color of the decaying wood; the partial destruction of the 
cellulose walls accounts for its crumbly nature. 

Make an enlarged drawing of a cross-section of the affected wood to 
show all changes which have occurred and the location and character of 
the mycelium. 

REPORT 

1. Give the method of treating the fence posts and explain the 
philosophy of the treatment. (See Farmers' Bui. 387.) 



STIFFEN 

This disease goes under a variety of names; Baldwin-spot, bitter pit, 
and fruit-pit being the most commonly tised. It is one of the non-parasitic 
diseases. It occurs the world over, wherever apples are grown. 

SYMPTOMS 
So far as is known, lesions are evident only in the fruit. Examine 
the specimicns provided, studying carefully the external characters of the 
lesions, observe: — 

1. The slightly depressed areas or "pocks"; their size, color and 
shape. 

2. Their distribution over the fruit, usually most ntmierous in 
what region? 

3. The differences between these lesions and those of the New 
England fruit-spot, specimiCns of which are provided. 

Make a sketch of an affected apple to show the general character and 
distribution of the lesions. 

Make an enlarged detailed drawing of a lesion of the stippen and 
beside it one of the New England fruit-spot for comparison. Label 
fully. 

Divide the apple (with the scalpel) lengthwise through the core, ob- 
serve : — 

4. The internal appearance of the lesion; color, size and shape. ■ 

5. The distribution of the lesions; relation to the surface-pits 
and to the vascular bundles. 

SKETCH the cut surface to show the form, size and distribution of the 
lesions as seen in this aspect. 

Placing the halves together, cut the fruit across the core, study: — 

G. The character and distribution of the internal lesions as 
exhibited in this aspect, especially their relation to the vascular strands. 
SKETCH to show the lesions as seen in the cross-section of the fruit. 

Study the individual internal lesions more critically as to : — 

7. Usual form, color, texture and taste. Compare with the 
adjacent healthy tissues. Are there any indications of a bitter taste? 

ETIOLOGY 

Many theories have been advanced with respect to the cause of stippen. 
Pathologists appear to be generally agreed that it is due to extreme varia- 
tions in the water-supply to the tree during the growing-season. (For a 
full discussion of etiology see the text or McAlpine, Bitter Pit Investiga- 
tions. 1:110; 2:93-95; 4:73-75.) 

Pathological Histology. The effect on the host-tissue is necrotic. 
Select one of the lesions next to the skin of the fruit which shows no cavity 
as yet. Make numerous thin sections with a sharp razor through the 
lesion at right angles to the surface of the fruit. Float these at once in 
clean water in a watch-glass. Mount one of the thin but entire sections. 
Cover and study: — 

8. The diseased cells; their collapsed walls, contents, and color 
as compared with the adjacent healthy cells. Treat the section with 
iodine-solution and examine for starch. Where found and why present? 

107 



108 

9. The relation of the diseased cells to the vascular cells and 
vessels. 

DRAW to show the points brought out in 8 and 9. 

REPORT 

1. Write a full but concise description of the symptoms of 
stippen and characterize briefly other fruit-spots with which it may be 
confused. 

2. Detail a plan of procedure by which a fruit-grower may hope 
to reduce the stippen to a minimum in his orchard. 



.. HYPOPLASTK DISEASE! 



DOWNY MILDEW OF GEAPES 

This is a common disease of both wild and cultivated grapes. It is in 
some years very destructive to certain varieties in American vineyards 
but is far more destructive to cultivated grapes in Europe. It also affects 
the five-leafed ivy, Psedera quinquejolia (L.) Green. 

SYMPTOMS 
This disease affects the leaves, young canes and fruit of the grape. 
On the leaves. Examine the leaves of cultivated grapes provided and 
OBSERVE : — 

1. The location, size, color and general appearance of the spots 
on both surfaces of the leaf. 

2. Difference in the appearance of the old and young lesions, 
especially on the upper surface. The tissues are not directly killed, but 
show a brownish yellow color contrasting sharply with the green of the 
healthy parts. (See autochrome 1.) 

3. The downy white growth covering the under surface of the 
lesion, — the fruit-structures of the pathogene. 

4. The above characters as exhibited by the lesions on the smooth 
leaves of the wild grape. 

Make sketches of both sides of affected leaves to show the characters 
of the lesions as obser\^ed. 

On the canes. The carie-lesions may be either local or systemic. 
Study the specimens showing local invasion, observe: — 

5. That the lesions are usually on one side of the stem, causing 
it to bend or curl, the diseased area being on the outer side of the curv^e. 

6. A slight increase in the size of the affected portion of the cane. 

7. The white downy growth of the pathogene, covering the 
lesion in many cases. 

SKETCH to show a localized stem-lesion. 

Examine the specimens (illustration specimens) and photographs of 
systemically invaded shoots, observe: — 

8. The distinct dwarfing or hypoplasia of the shoot and its organs, 
— the leaves and tendrils. 

9. The continuous white coating of the fruiting structures of 
the fungus. 

SKETCH a portion of a shoot to show these characters. Only very 
young shoots show systemic lesions. 

On the fruits. Examine the specimens and photographs provided. 
observe: — 

10. That certain of the berries in the bunch are covered with the 
downy white growth obser\'ed on leaf and stem -lesions. 

11. That the affected berries are brown in color, in striking 
contrast to the green healthy fruits on the bunch. The disease is, on this 

109 



110 

account, sometimes known as "brown rot." Compare with grape affected 
with anthracnose or black rot. (See illustration specimens.) 

Sometimes, particularly in the older berries, the fungus does not seem 
to be able to send forth the white downy fruiting structures. Such berries 
fail to ripen and have a whitish opaque color. This character is not well 
seen in preserved specimens. 

SKETCH a bunch of grapes to show the contrast between diseased and 
healthy berries. 

ETIOLOGY 

The fungus which causes the downy mildew is a Phy corny cete known 
as Plasmopara Viticola (Berkley and Curtis) Berlese and De Toni. It is 
a native of America and was introduced into Europe about 1878 where it 
has since wrought great destruction. 

Life-history. Thanks to the extensive investigations devoted to this 
pathogene, our knowledge of its life-history is now relatively complete. 

The Primary Cycles are initiated shortly after growth of the host 
starts in the spring. The sources of the primary inoculum are in the pre- 
viously diseased overwintered leaves on the ground. 

Pathogenesis. Examine bits of overwintered leaves or prepared 
mounts of the same, search carefully for: — 

12. Rather large, globose bodies, — the oospores, embedded 
in the tissues of the leaf. They are readily distinguished by the circular 
band-like appearance of the thick hyaline inner wall. These are resting- 
spores which serve to carry the fungus through the winter. 

These germinate during wet weather in the spring in the old leaves as 
they lie on the ground. The study of actual germination of these spores 
is attended with difficulties; therefore, study separates of Gregory's article 
from Phytopath. 2:237, fig. 2. observe: — 

13. The large rough oospores embedded in the disorganized 
leaf-tissues. 

14. The slender stalk, — conidiophore, sent forth from a crack 
in the wall of the oospore and bearing at its tip a large egg-shaped conidium 
(sporangium). 

COPY, fig. 2 a and b. Label fully. 

When mature, this conidium is readily broken off by the breeze or splash- 
ing rain-drops and carried to young leaves or growing canes. Here in a 
drop of water, the conidiiim, a potential sporangium, germinates by the 
division of its protoplasm into (usually) 5-8 swarmspores, which are 
emitted through the papillate apex as shown in Gregory's article, fig. 3. 

If viable conidia are available, attempt to germinate them, proceeding 
as directed by the instructor. Watch the process carefully. 

Make drawings, either from observed germination or from Gregory's 
drawings, to show swarmspore-formation. 

These swarmspores swim about by means of two flagella (Gregory's 
figures 4c and 7). copy drawings to show swarmspore-germination and 
invasion through a stoma. 

From the swarmspore-germtube, a mycelium is developed which rami- 
fies the tissue, passing between the cells. 

Make thin tangential sections through the cortex of a diseased cane or 
berry. Motmt in chloral hydrate to clear, observe : — 



Ill 

15. The large granular mycelium, fitting so closely into the 
intercellular spaces that its walls are not readily distinguished from those 
of its host-cells ; septate or non-septate ? 

16. The numerous small globose haustoria extending through the 
walls of the host-cells. They push the plasma-membrane inward but do 
not penetrate it. 

Make a drawing of the mycelium showing its structure and relation 
to the host-ceUs. 

This mycelium soon develops special branches which are sent forth 
through the stomata and branching, form the conidiophores. These 
conidiophores constitute the downy white growth on the under surface 
of the lesion. Scrape some of the conidiophores from the specimen pro- 
vided for this purpose. Mount in potassium hydroxide. Examine and 
OBSERVE : — 

17. Their color, fonn of branching and shape of the ultimate 
tips or branchlets on which the conidia (sporangia) are borne. Estimate 
carefully the number of conidia that may be borne on each conidiophore. 

18. The egg-shaped conidia lying all about ;n the mount. 
They separate from the conidiophore very readily when mature. By 
which end were they attached? A mount from near the margin of a 
young lesion may show the immature conidia still attached to the conidio- 
phores. 

Study sections through a leaf -lesion, observe: — 

19. The relation of the conidiophores to the host-structures; 
emergence through the stomata; number from each stoma; and constric- 
tion at the stomatal opening. 

Make a large drawing of a conidiophore with mature conidia attached 
to a few of the branches. 

These conidia are disseminated by the wind. They fall upon leaves, 
fruits and stems and initiate secondaiy cycles. 

Saprogenesis. The mycelium within the dying leaf-tissue begins 
the sexual formation of long-lived resting-spores to carry the pathogene 
through the winter. 

The early stages in the development of these oospores are not easily 
studied in the case of Plasmopara Viticola, but may be readily observed 
in pure cultures of related phycomycetous fungi. Make mounts from the 
culture provided and observe : — 

20. The large densely granular swollen tips of many of the my- 
celial branches, — oogonia or female organs. The cross-wall cutting off 
some of the older ones from the mycelium. 

21. The much sm.aller granular body, also a modified mycelial 
tip, apphed closely to the side of the oogonium, — ^the antheridium or male 
organ. From this a fertilization-tube is sent into the oogoniixm and a 
nucleus passes through to fuse with a female nucleus in the oogonium. 
The contents of the fertilized oogonium now round up to form a single 
oospore. 

DRAW to show the early stages in oospore-formation. 

Oospore-formation may probably begin while the mycelitun is stiU 
drawing nutriment from the living host-tissues. The oospores are, 
however, not matured until after the tissues in which they are formed 
are dead. The oospores mature in the fallen leaves on the ground. Study 
the mature oospores provided, observe : — 



112 

22. The thin outer hyaline sack, — the old oogonial wall. Is there 
any trace of the old antheridium? 

23. The uniformly thick hyaline inner wall surrounding the 
oily protoplasm, — the living part of the oospore; why oily'' 

24. The dark colored irregularly thickened outer wall enclosing 
the inner wall. This consists of worthless unused protoplasm contracted 
about the inner wall as a protection. 

The thicker inner wall serves as a stored food-reserve for the germinating 
oospores. It dissolves from within as the genninating spore develops 
the long conidiophore with the primary conidium (already studied). 

DRAW to show the structure of a mature oospore. 

Secondary Cycles are initiated repeatedly during the season on leaves, 
stems and fruits by the conidia from primary and other secondary lesions. 
These conidia germinate, as do the primary conidia, by swarmspores. The 
secondary cycles repeat in all details the phenom.ena of the primary cycles. 
Only in the leaves, so far as known, are the cycles completed by the forma- 
tion of oospores. 

REPORT 

1. Describe in detail two methods, one an eradicatory, the other 
a protective method for the control of the downy mildew of grapes, and 
state conciselv whv each should be effective. 



DOWNY MILDEWS OF TME RANUNCULACEAE 

There are two downy mildew diseases of the Ranunculaceae in America, 
the Plasmopara mildew of anemones and hepaticas and the Peronospora 
mildew of buttercups. While at present it is chiefly the wild species that 
suffer, these diseases constitute a serious menace to the development 
of garden-varieties from the wild species. 

SYMPTOMS 

The symptoms of the two diseases are very similar. The lesions are 
of two general sorts, localized and generalized or systemic, that is, involv- 
ing the entire plant or shoot. It is chiefly the leaves that exhibit the 
effects of the disease. 

Local lesions. Examine the affected leaves of the different hosts 
provided, observe : — 

1. The pale-gray or brownish color of the upper surface of the 
affected parts, in striking contrast to the green unaffected areas, especially 
noticeable in the anemones; the angular shape of the lesions; to what due? 
Note that nearly the entire blade may be involved, sketch. 

2. The white felty coating covering the lesions on the under 
surface of the anemone leaves; violet-gray on the buttercup leaves. 
This felt is composed of the conidiophores and conidia of the pathogene, 
and constitutes the most distinctive sign of these downy mildew diseases. 
(Compare with grape mildew, illustration specimens.) sketch to show 
the appearance of the lower surface of the lesion. 

Systemic lesions. The hypoplastic or dwarfing effect of these diseases 
is especially well observ^ed in the individuals which harbor the living 
pathogene in the rootstocks during the winter. The entire leaves from 
such rootstocks, at least the earlier leaves, are affected. In the specimens 
provided, observe: — 

3. A marked dwarfing of diseased as compared with healthy 
leaves, especially striking in the buttercups. 

4. The more greyish color of affected leaves. This is very strik- 
ing in affected x^lants of Ranunculus acris L. in early spring. 

5. The continuous coating of conidiophores on the lower surface. 
The affected leaves or portions of leaves soon die and turn black, shrivel 
and become brittle. 

Make sketches of diseased and healthy leaves to show contrast in 
size. 

ETIOLOGY 

The disease in anemones and hepaticas is caused by Plasmopara pyg- 
maea (linger) Schroeter. while Peronospora Ficariae TulavSne attacks 
various species of Ranunculus. They are closely related phycomycetous 
fungi. 

Life-history. The activities of these two pathogenes throughout their 
life-cycles are essentially alike. They develop the same kind of organs 
and stnictures which are, however, distinctive for each parasite. 

Primary Cycle. These pathogenes are obligate parasites, the period 
of saprogenesis being probably one of rest and of development at the ex- 
pense of stored food. 

113 



114 

Pathogenesis. There are two quite different sources of primary 
inociilum; one the dead overwintered leaves, diseased the previous season; 
and the first leaves put forth from systemically invaded plants. 

(a) Inoculum from overwintered leaves. Overwintered hepatica leaves, 
showing last year lesions, have been cleared by special treatment and 
placed in alcohol in vials. Holding one of these to the light, examine with 
the hand-lens and observe : — 

6. The great mimber of minute globose dark-colored oospores 
imbedded in the tissues, sketch a portion of the leaf to show these. 

These germinate in situ, sending forth from each oospore a slender 
conidiophore bearing one or more conidia. Oospore-germination is not 
readily obtained. It probably does not differ materially from that des- 
cribed by Gregory for the oospores of Plasmopara Viticola in grape leaves. 
(Study Gregory's article, separate from Phytopath. 2:237, fig. 2). copy 
and label to explain the use of these figures. 

Borne by wind or splashed by rain-drops, these conidia reach the 
developing leaves of their respective hosts and initiate the primar}^ infect- 
tions. 

(b) Inoculum from systemJcally invaded plants. The pathogene 
winters as mycelium in the rootstocks of such plants. As the first leaves 
develop the mycelium develops with them and shortly after the leaves 
unfold, sends forth numberless conidiophores through the stomata on the 
under surface. These conidiophores develop conidia (primary inoculum) 
in every respect like those produced from the oospores. Mount some 
of the down from the under surface of a systemically invaded leaf. 
observe : — 

7. The mnnerous globose or egg-shaped conidia; their thin walls 
and densely granular contents, draw. Carried by the slightest breeze, 
these conidia fall upon young leaves of their host-plants and, if moisture 
is present, germinate and infect the plants. 

Conidia, whether produced from oospores or from mycelium from 
within the living leaf, germinate in the same way. If they are conidia of 
Plasmopara they form swarmspores. If they are conidia of Peronospora 
they germinate by a germtube. If viable conidia of either or both .are 
available, study germination and draw to show the structures produced 
or copy drawings provided by the instructor. 

Mycelium is rapidly developed from the germtubes of conidia or swarm- 
spores and spreads through the tissues. 

Make thin sections of diseased leaves of anemorife or hepatica (if dry 
leaves, mount sections in potassiiim hydroxide) and study carefully to 
locate : — 

8. The mycelium.. Is it intercellular or intracellular? Are 
haustoria present ; what form 'l 

9. Mycelial branches projecting forth throvigh the stomata; 
how many through each stoma? The form of these conidiophores is 
best studied in material scraped from the under surface of a lesion. Scrape 
a bit of the white felt from the under surface of one of these lesions on 
anemone or hepatica leaves, mount in xjotassium hydroxide and examine 
with the low-power, observe : — 

10. The short, rather stout, scarcely branched conidiophores. 
Do not confuse them with the long pointed thick-walled leaf-hairs. It 
is from the unusual shortness of the conidiophores that this fungus gets 



115 

its specific name, pygmaea. It has the pygmy conidiophores among the 
species of Plasmopara. Make a mount of Peronospora Ficariae from 
Ranunculus leaves provided, along with some of P. pygmaea in the same 
drop of water so that they can be compared as to size, form and branching. 
LOCATE : — 

11. A single entire conidiophore of each species well isolated in 
the mount. Study it carefully as to structure, thickness of walls, branching 
and arrangement of ultimate branchlets on which the conidia are borne. 
Make a large sketch of an entire conidiophore of each. Try to find in 
the mount a conidiophore with conidia still attached. Mounts from 
young leaves or the margins- of lesions will often give conidiophores with 
conidia attached. If one can be found, study a branch to determine the 
manner of attachment of the conidia. 

Study the mature conidia scattered through the mount, observe: — 

12. Their form; the slightly raised apical papilla to be dis- 
cerned on some of them; the thin wall and granular contents, draw to 
show the form of the conidia of the two pathogenes. 

These conidia, produced in great abundance, are carried by the wind 
to healthy plants during rainy weather, initiating secondary cycles. 

Make an enlarged drawing of a section through a leaf of one of the 
hosts to show the pathogene-structures within and without the leaf. 

Saprogenesis. As the diseased tissue begins to die, the mycelium 
of the pathogene produces branches, the tips of which swell and round up 
between the cells to form oogonia and antheridia. Fertilization takes 
place and before the dead and fallen leaves begin to disintegrate, oospores 
are matured. 

(To study the sexual structures and development of the oospore, follow 
the procedure outlined under Downy Mildew of Grapes on p. Ill, No. 20 
and 21.) 

Oospores of Plasmopara pygmaea are x^roduced in great abundance 
in leaf-lesions in Hepatica triloba as already observed under No. 6. To 
understand the strticture of the mature oospores and their relation to the 
tissues, examine prepared slides and make mounts from leaf-tissue mace- 
rated in potassium h\^droxide. observe: — 

13. The numerous dark-brown bodies imbedded in the tissues. 

14. The oospore proper, with its uniformly thick smooth hyaline 
inner wall, and its outer brown wall, irregular in thickness; enclosing the 
oospore, the transparent thin old oogonial w^all usually collapsed tightly 
against the outside of the oospore; remnants of empty hyphae. As these 
oospores are nearly mature, no trace of antheridia will probably be found. 

DRAW carefully to show the structure of a mature oospore, much en- 
larged. 

These oospores germinate in the spring as they lie in the old leaves on 
the ground, each giving rise to a conidiophore with conidia and so provide 
the primary inoculum as already seen. 

Secondary Cycles are initiated by the conidia produced during the 
primary cycle. Ordinarily, as in the case of the primary lesions, local 
lesions result. In the case of the secondary cycles initiated late in the 
season, the mycelium arising from the germ tube may. instead of causing 
a local lesion, spread throughout the stemi and root of the plant without 
killing it. It becomes perennially associated with the tissues of the living 
host. It grows up into the new leaves put forth in the spring and sends 



116 

forth from their entire under surfaces, conidiophores bearing conidia. 
Such invaded plants are said to be suffering from "systemic infection". 
(See text.) 

Systemic infection is very common in the case of Peronospora Ficariae 
in the buttercup, Ranunculus acris. Examine one of the diseased leaves 
of R. acris. note: — 

15. That the conidiophores cover the entire under surface of 
the leaves. Where this occurs, one may be quite sure it is a case of "sys- 
temic infection" and not a local lesion. 

If fresh m^aterial is available, make sections through some part of 
systemically infected plants (crown or rootstock). Stain with methyl blue, 
wash thoroughly, cover and locate the mycelium in the tissues; inter- or 
intracelliilar ? Haustoria? draw. 

The mycelium continues to live in the gradually weakening host, pro- 
ducing one crop of conidia each season from which primary infections, 
local or systemic, in character may arise. It eventually perishes with 
the host. 

REPORT 

1. If a gardener discovered some of his perennials to be suffering 
from systemic infection, what methods of control should he employ? 
Why? If all the infections are local, what should be the treatment? 
W^hy? 

2. Show in a graphic diagram the life-cycles of either of the 
pathogenes studied in this exercise. 



POWDERY MILDEWS OF FLORISTS' CROPS 

Powdery mildew diseases frequently affect various ornamental plants 
of greenhouse and garden. They are sometimes very destructive and 
commonly troublesome. Among such plants which most often suffer from 
the powder}^ mildews are roses, phloxes, sweet peas, willows, hawthorns, 
lilacs, honeysuckles, bittersweet and dogwoods. 

SYMPTOMS 

The leaves are usually the organs affected, although stems, blossoms 
and even fruits may be diseased. Powdery mildews are usually most 
common and conspicuous in gardens and borders toward the latter part 
of the season. The white mealy coating which is formed on the affected 
organs is usually very striking. The minute black perithecia of the patho- 
gene frequently appear in great numbers late in the summer or early au- 
timm, in some cases standing out sharply against the white mycelial mats 
on which they rest. Where the mycelium is sparse or webby, the black 
perithecia may not be easily detected. In many cases they are but rarely 
formed. A tendency to stunt or dwarf the host is commonly to be ob- 
served. This is much more striking in some cases than in others. 

On the rose. Examine the diseased shoots provided, observe: — 

1. The white felt, covering large areas on the canes and running 
out over the thorns; in some cases localized about the base of large thorns. 

2. The powdery and less felty character of the white coating of 
the leaves. Which surface is affected? 

3. The curling and dwarfing effect on the leaves, especially 
marked in hothouse-roses and in ramblers. 

4. The abnormal coloration sometimes exhibited by the leaf 
under and about the mildewed spot. 

5. The dwarfing of the entire tips or branches of some shoots, 
most frequently observed in ramblers. This results from bud-infection 
explained later. 

6. The white mycelial felt, coating young buds and hips. The 
buds are often so stunted that they fail to open or the affected hips are 
dwarfed and do not ripen. 

Make drawings to show the symptoms exhibited in the material 
studied. 

On phlox. All above-ground parts of this host are likely to be affected. 
The mildew-spots are most prominent on the leaves. Examine the speci- 
mens provided, observe: — 

7. The felty white mycelial patches on the leaves. How do 
the patches on the two surfaces of the leaf differ? 

8. The purple coloration often developed beneath and about 
the spots. 

9. The yellowish color of the mycelial mat in the older patches 
and their tendency to coalesce and cover the larger part of the leaf -surface. 

10. The brown centers of many of the spots due, as may be seen 
with the hand-lens, to the perithecial fruit-bodies of the pathogene. 

11. The mycelial patches on stems and inflorescence; less felty, 
often hardly discernible but usually covered with the brown perithecia. 

117 



118 

12. The dwarfing effect on the inflorescence. Flowering is 
often partially or entirely prevented. 

Make a series of drawings to show the symptoms exhibited by mildewed 
phlox. 

On peas. Both sweet peas and garden- or field-peas are affected. 
Examine the material provided and note : — - 

13. That the mycelial coating spreads almost tmiformly over 
the entire leaf-surface, "stems and pods. 

14. That it is much thinner than that on rose or phlox, and is 
web-like instead of felty. 

15. That there is little difference in the character of the mycelial 
coating on the upper and lower surfaces of the leaf. 

16. The general effect on the growth of the plant. Is dwarfing 
marked? Peas are usually affected late after growth is largely completed. 

17. The minute black perithecia in groups here and there in the 
mycelial weft; not prominent; usually not found on sweet peas. 

Make drawings to show the powdery mildew on peas. 

On lilacs. The powdery mildew on the lilac is so common as to be 

alm^ost always found on lilacs wherever grown and every year. The leaves 

are the organs affected. Examine the material provided and observe : — 

IS. The character of the mycelial coating. On which surface of 

the leaf does it develop? 

19. The mjinute black perithecia; their arrangement and 
distribution on the leaf. 

20. Any evidences of pathological effects on the leaves. 
Make drawings to show the symptoms of the mildew on lilac leaves. 

On bittersweet. This mildew is not only common on Celastrus scan- 
dens L. but affects the foliage of many shrubs and trees. Examine the 
leaves provided and observe : — 

21. The size and location of the spots ; the habit which this parti- 
cular mildew-pathogene has, of sending mycelial branches into the leaf- 
tissues through the stomata, is responsible for the location of the spot. 

22. The chlorotic effect on the leaf -tissue beneath the mildewed 
area as evidenced through the -upper surface. 

23. The character of the superficial mycelial grow^th. 

24. The comparatively large and nrmierous perithecia in all 
stages of development, the younger ones smaller and brown or yellow in 
color. 

DRAW to show a leaf with a mildewed spot. 

ETIOLOGY 

Powdery mildew pathogenes all belong to the Erysiphaceae, a family 
of ascomycetous fungi. They are characterized among other things by 
their habit of growing externally over the surface of their hosts. They 
attach themselves by means of short haustoria sent into the epidermal 
cells. One or two species are known to send intercellular hyphae through 
the stomata into the tissues. The diseases above studied and their 
respective pathogenes are: — the powdery mildew, 

of rose, caused by Sphaerotheca pannosa (Wallroth) Leveille; 

of phlox, caused by Erysiphe Cichoracearum DeCandoUe; 



119 

of peas, caused by Erysiphe Polygoni DeCandolle; 

of lilac, caused by Microsphaera Alni (Wallroth) Winter; 

of bittersweet, caused by Phyllactinia Corylea Karsten. Besides 
the genera above represented, two more, Uncinula and Podosphaera 
are known, species of which occur on trees or shinibs of the yard 
and garden. Examples: — Podosphaera Oxyacanthae (DeCandolle) de 
Bary, on species of Crataegus, Prunus, Spirea and others; Uncinula Salicis 
(DeCandolle) Winter, on species of Sahx and Populus. 

Life-history. The powder^' mildew fungi exhibit such similarity in 
structure and life-habits that the same outline will serve for the study of 
the life-history of any of them. From. this point the student will follow 
the outline as given for the Powdery Mildews of Trees and Fruits, p. 125, 
including the subject designated for "the report. 



POWDERY MILDEW OF CEREALS AND GRASSES 

This is a very common and sometimes serious disease of wheat, rye 
and other cereals. It is also to be found commonly on various wild grasses, 
especially species of the genus Poa. 

SYMPTOMS 

The powdery mildews are detected chiefly by the pathogene-structures 
developed upon the exterior of the host. There are also some accompany- 
ing affects or symptoms exhibited by the host. The lesions are confined 
largely to tlie leaves and leaf-sheaths. In the material provided, 
OBSERVE : — 

1. The densely matted white, sometimes brownish, mycelial 
patches on the surface of the leaf (upper and lower). In the fresh condi- 
tion these patches are powdery due to the abundance of conidia, hence the 
name, powdery mildevv'. 

2. On the wheat leaves especially, the minute black bodies 
buried in the mycehal mass, usually most abundant at the center of the 
spot. These are the perithecia of the pathogene. 

3. The effect on the tissues of the leaf beneath and about the 
mycelial mat. Note that in some cases the entire leaf has turned brown 
and died. 

Make sketches showing these symptoms. 

Where the attack is severe, there is a dwarfing of the heads or a shriveling 
of the maturing grains, or both. If material is available, study and com- 
pare with healthy heads and grains. Make sketches to show the compari- 
son. 

Examine the illustration specimens of powdery mildews on various hosts 
provided and note: — 

4. The marked similarity in the sjanptoms exhibited by all of 
them. Select one of the specimens and sketch. Label fully. 

ETIOLOGY 

The powdery mildew of cereals and grasses is caused by Erysiphe 
graminis DeCandolle, the conidial form of which is known as Oidium 
monilioides Link. Like all the other powdery mildew pathogenes it is 
an ascomycetous fungus belonging to the family, Ery.siphaceae. They 
all develop externally iipon the surface of their host except for short 
haustoria sent into the epidermal cells or, in the case of one or two species, 
an occasional mycelial thread sent through stomata into the tissues. 

Life-history. This pathogene exhibits during its life-cycles all of the 
characteristic structures of the powdery mildew fungi. 

The Primary Cycles are initiated in the spring. The sources of 
inoculum are the overwintered perithecia on the leaves and stubble of the 
host. 

Pathogenesis. Remove some of the minute black perithecia 
from the mycelial mats on the old overwintered host-leaves. Crush in a 
drop of water by pressing on the cover-glass, observe : — 

5. The large ellipsoidal ascospores forced from the perithecia; 
some still in the asci. Determine the number of asci in each peri- 

120 



121 

thecium. These constitute the primary inoctilum. draw to show the 
form and structure of these ascospores. 

When the ascospores are mature and the perithecium is thoroughly'' 
wetted, it cracks open and the ascospores are forcibly discharged. Borne 
by the wind, thc}^ fall upon the growing leaves of the host and germinate. 
If viable ascospores are available, study spore-germination as seen in the 
slides provided, draw, or copy illustrations provided by the instructor. 

As soon as this germtube has developed a food-relation with the host 
by means of haustoria in the epidermal cells, a mycelium begins to develop, 
branching and spreading in all directions over the leaf-surface. Examine, 
under the binocula rmicroscope, one of the white mycelial mats on the 
diseased green leaves (fresh or preserved) provided and observe : — 

6. The tangle of silver-white hyphae with long spreading branches 
about the margin of the lesion. 

7. The numerous erect chains of conidia borne on short conidio- 
phores. Many of these conidia have fallen ofi and give the mealy appear- 
ance to the mildew-spots. 

Scrape the mycelial mass from the surface of the leaf. Mount in water, 
cover and examine : — 

8. Conidia; large ellipsoidal, flattened slightly at the ends; 
their thin hyaline walls and densely granular protoplasm. 

9. Conidiophores ; short, with a swollen base just above the 
point of attachment to the mycelium. Try to find a conidiophore with 
several conidia still attached. 

10. The mycelium; very crooked and much broken in scraping 
from the leaf ; septate or nonseptate ? 

As the mycelium spreads over the surface of the leaf it sends minute 
branches through the outer cell-wall of the epidermal cells. This branch 
enlarges and branches within the cell to form the finger-like haustoria. 
Studv these in the slides provided or from the drawings bv Smith, Bot. 
Gaz.'ZQ, pi. XI and XII. 

Make a composite drawing to show a cross-section of the epidermal 
cells of the host with haustoria, mycelium, conidiophores and conidia in 
normal relation to each other. 

These conidia break off at the top of the chain as fast as matured and, 
scattered by the wind, initiate secondary cycles. 

Conidia continue to be produced for a time by the myceliimi on the 
primary lesions. As the primary lesions are largely on the young or 
seedling-leaves of the host, the myceliiun probably perishes along with the 
young leaf before the sexual fruit-bodies can be developed. These appear 
later on the mycelium of the secondary cycles. There is, therefore, no 
saprogenic phase in the primary cycles. 

The Secondary Cycles are initiated by conidia from the primary 
cycles. 

Pathogenesis . The fungus exhibits the same conidial structures in the 
secondary cycles as those just studied. As the host-tissues begin to mature, 
conidial production ceases, and the mycelium begins the development of 
sexual structures. The detailed study of the development of these struc- 
tures cannot well be followed out in this laboratory exercise. (See de 
Bary, Morphology and Biology of the Fungi, p. 226, fig. 107.) 

The structure of the peritheciuin may, however, be readily studied. 
Examine the specimens provided under the binocular microscope, ob- 
serve : — 



122 

11. The much coarser and more densely matted mycelium; 
not pure white but yellowish or brownish. 

12. The globose perithecia of varying sizes and colors enmeshed 
in the mycehal mat. 

SKETCH to show the appearance under the binocular microscope. 

Sapr agenesis. The perithecia usually begin to appear while the 
leaf is still green, but do not mature their ascospores until the leaves 
die and are overwintered. The perithecia on the dead leaves on the ground 
pass the winter in an inactive condition. The rains and warm weather of 
spring cause the asci to mature their ascospores. 

To simulate the spring conditions, some of the leaves bearing im.mature 
perithecia have been placed in v/arm water for several days. Thej'' are 
now mature. 

Remove some of the perithecia to a slide in water and cover with a cover- 
glass. Crush by pressing firmly on the cover with the handle of a scalpel, 
while watching the perithecia under low-power. What comes out of the 
perithecitmi ? How many? What is the character of their contents? 
Does the perithecium have an ostiolum? 

DR.vw to show a peritheciiun with its contents. Show the structure of 
the perithecial wall and mycelial attachment. 

With the maturity and discharge of the ascospores, the secondary 
cycles are completed. 

REPORT 

1. E.Kplain the significance of biologic strains in E. graminis 
DC, with respect to control. 

2. Discuss eradication measures in the control of powdery 
mildews. 



POWDERY MILDEWS OF TMEES AND FMUITS 

Powdery mildew diseases have been reported on about 1500 species of 
wild and cultivated plants. Several of them are frequently very injurious 
to fruit-trees and sometimes to shade- and forest-trees. 

SYMPTOMS 

Leaves and young shoots are usually the parts of the host that are 
affected. Throughout the latter part of the siimmer the powdery mil- 
dews are conspicuous and give to the infected parts of the host-plant a 
whitish, mealy or dusty appearance, due partly to the superficial white 
web-like mycelitim of the pathogene, and partly to the presence of myriads 
of rapidly formed white conidia. Later in the summer, and in the autumn, 
there usually appears, on the affected parts, the small black spherical 
perithecia of the sexual stage. In the autumn these are more in evidence 
than the whitish growth; the latter often disappears. These signs, the 
fruit-bodies of the pathogene, are usually the most striking evidences of 
the disease. Definite and characteristic symptoms resulting from the 
effects of the pathogene on the host are, however, not wanting in many 
cases. 

On the cherry. On this host, the leaves and twigs show the effects 
of the disease. Examine the specimens and photographs provided and 
OBSERVE : — 

1. The upward rolling or curling of the leaf -blades parallel with 
the mid-rib. 

2. The shorter and thicker internodes of diseased twigs as com- 
pared with healthy ones. 

3. The weft -like coating of fine white hyphae on the leaves, 
especially on the under surface. The mycelium of most of the powdery 
mildew pathogenes is entirely superficial . 

4. Patches of the mycelial weft dotted with the minute black 
perithecia of the pathogene. 

Make a drawing of a diseased tv/ig to show the signs and symptoms 
exhibited. 

On the apple. The young leaves, flower clusters and shoots are af- 
fected. Examiine the diseased shoots and U. S. Agr. Dept. Bui. 120, 
pi. I and VI, provided, observe: — 

5. The marked hypoplastic effect exhibited in the dwarfed 
foliage. 

6. The mealy white coating of the diseased leaves, — conidia 
and mycelium of the fungus. 

7. The thick felty mycelial coating on the watersprouts collected 
in the autmnn. Note the dirty white or brownish tinge as compared with 
the pure white of the growth on the leaves. 

8. The minute perithecia, more or less embedded in the mycelial 
mat on the shoots. , 

Make drawings to show' the appearance of affected leaves and water- 
sprouts. 

On the peach. Not only the leaves and twigs, but also the fruits of 
the peach, are subject to the disease. Study the specimens and photo- 
graphs provided, observe : — 

123 



124 

9. That the leaves are narrow, have failed to expand and are 
curled and deformed. Chlorophyl is not developed properly and the leaves 
show red and yellow tints. Defoliation often results from a severe infec- 
tion. 

10. The effect on the more succulent upper parts of the twigs. 

11. The felty character of the superficial mycelium which forms 
in white patches over affected leaves and twigs. This thick mycelial 
felt persists on the twigs after the leaves fall, becoming a dirty gray-brown 
in color. 

12. The more or less circular white patches of mycelium on the 
fruit. When very young fruits are affected they soon fall. 

Perithecial fruit-bodies rarely appear. 

Make drawings to show the symptoms exhibited by the powdery 
mildew of the peach. 

On the grape. This disease is more destructive and develops more 
typically in the Pacific Coast regions than in eastern United States. All 
herbaceous parts of the host are affected. Examine the specimens pro- 
vided. OBSERVE : — 

13. The whitish patches on the upper and lower surfaces of 
the leaf. How do they compare with the spots of the downy mildew? 
These spots may spread to form a whitish, m.ealy coating over the greater 
part of the leaf-surface. Badly diseased leaves may curl upwards about 
the edges. 

14. The small black perithecia scattered over affected areas on 
the leaves. 

15. The diseased canes. The^/ also show the superficial greyish 
white patches of myceliiun beneath which the tissues of the cane soon 
darken, making it spotted. (See California Bui. 186, fig. 3.) 

16. The diseased berries. (See California Bvl. 186, fig. 4.) 
Blossoms and young fruits when affected, quickly fall. The disease may 
often cause shelling of the large green berries when the fruit-pedicles are 
affected. 

Make drawings from specimens and illustrations to show the symptoms 
of the powdery mildew of grapes. 

On the gooseberry. In case of the gooseberry mildew, it is chiefly the 
young shoots and the fruits that are affected. In the material provided, 
observe : — 

17. The mycelial m^ats coating the shoots; color, thickness 
and distribution. 

18. That the superficial myceliiun spreads out over the leaves. 
How is the growth of leaf and stem affected? 

19. The character of the mycelial patches on the fruits. Does 
the growth and development of the fruit appear to be affected ? 

20. The black perithecia of the pathogene embedded in the 
mycelial felt. 

This same gooseberry mildew may sometimes seriously affect some 
varieties of currants, as may be seen in the specimens provided. 

Make drawings of mildewed shoots and fruits of gooseberry or currant. 

On the chestnut. The leaves are the organs affected. This powdery 
mildew affects not only chestnut but a great variety of trees, shrubs and 
woody vines. Exainine the diseased leaves provided, observe: — 



125 

21. That the mildew-patches are confined to the under stirface 
of the leaf. 

22. The rather thick felty character of the mycelial growth; 
color and extent. 

23. The numerous perithecia sitting on the mycelial mat, 
not embedded in it; larger than the perithecia of the other mildew- 
pathogenes observ^ed; some of them immature as indicated by their small 
size and light color. 

24. Any evidence of injury showing on the upper surface op- 
posite the mildew-spot. 

DRAW a leaf showing the characters of the mildewed areas. 

On the willow. Many species of willow and also poplars are affected. 
The leaves are usually the only organs involved. In the specimens 
provided, observe: — 

25. The location and distribution of the spots; hypophyUous 
or epiphyllous ? 

26. The characteristic dense white mycelial border of the spot 
with darker center, especially in older lesions, due to the numerous black 
perithecia. Willows are often defoliated by this disease. 

27. Any evidence of injury to the tissues. 
DRAW a willow leaf showing the mildew-spots. 

ETIOLOGY 

The powdery mildew diseases are all caused by species of ascomycetous 
fungi belonging to a single family, the Erysiphaceae. Each disease above 
studied is caused by a different species of pathogene. Even within some 
of these species there are doubtless biologic species. The diseases studied 
with their respective pathogenes are as follows: — the powder)^ mildew, 

of cherry, caused by Podosphaera Oxycantkae (DeCandoUe) de Bary; 

of apple, caused by Podosphaera leucotncha (Ellis and Everhart) 
Salmon ; 

of peach, caused by Sphaerotheca pannosa (Wallroth) Leveille, var. 
Persicae Woronichin ; 

of grape, caused by Uncinula necator (Schweinitz) Burrill; 

of gooseberry and currant, caused by Sphaerotheca Mors-uvae (Schwei- 
nitz) Berkley and Curtis; 

of chestnut, caused by Phyllactinia Corylea Karsten; 

of willow, caused by Uncinula Salicis (DeCandolle) Winter. 

Besides the four genera represented, Podosphaera, Sphaerotheca, 
Uncinula and Phyllactinia, two more are known, species of which are very 
common in this country. Examples: — Erysiphe graminis DeCandolle, 
on grasses and cereals and Microsphaera Alni (Wallroth) Winter, on lilac. 
(See demonstration specimens.) 

Life-history. These powdery mildew pathogenes all exhibit such 
similarity in their structures and life-habits that the following outline 
should serve for the study of any of them. As they are strictly obligate 
parasites, saprogenesis is a period of rest or a maturation-process carried 
on at the expense of stored food-reserves. 

The Primary Cycles are initiated in the spring or early simimer. 
The primary inoculum is usually the ascospores from ovenAdntered peri- 
thecia. In some cases, as in that of 5. pannosa (Wallr.) Lev., or of Podo- 



126 

sphaera leucotricha (E. and E.) Salm., the mycelium may winter in a semi- 
dormant condition within the host-buds on the embryonic leaves. As 
these buds open and the leaves and shoots develop in the spring, the myce- 
liimi grows out over them and produces con idi a, which, scattered to healthy 
shoots, mav initiate primary infections. (Sec U. S. Agr. Dept. Bui. 
120:9-10.) 

Ascospores are, however, responsible for the primary infections in 
the case of most powdery mildews and often even in those in which hiber- 
nating mycelium is known. 

Pathogenesis. Remove with the scalpel, several perithecia of one 
of the pathogenes indicated by the instructor. Mount in a drop of potas- 
sium hydroxide. Cover and, while examining it under the low-power, 
crush the perithecium by gently pressing on the cover-glass with the point 
of the scalpel, observe: — 

25. The irregular crack in the peritheciuni from which one or 
m.ore asci are forced out. How many in this case? 

20. The large ellipsoidal ascospores usually remaining within 
the asci; number in each ascus; color; contents. 

27. The thin place in the wall of the ascus at the apex. At 
maturity this dissolves as the perithecium cracks open and the ascospores 
are forcibly ejected into the air. 

Make a diagrammatic drawing of a cracked perithecium with protrud- 
ing asci discharging spores. 

Bome by the breeze, these ascospores fall upon young shoots or leaves 
of the host and germ.inate. The germtube grows out over the surface 
and sends a haustorium into the epidermal cells, from which point the 
branching mycelium cievelops. Study germinating ascospores or illustra- 
tions provided, especially Bot. Gaz. 29, pi. XI-XII. Make a diagram- 
matic DRAWING of a germinating ascospore with haustorium. 

To study the mycelium, scrape some from the surface of young spots; 
tease apart in water or potassium hydroxide; cover and examine, ob- 
serve : — 

28. The broken pieces of irregular, branched mycelium; septa, 
color and contents. 

29. The large, ellipsodial or ovoid conidia; color and contents. 
Several may be found attached in a chain or even still on the conidiophore. 

30. The short conidiophores, — upright branches which bear the 
conidia in a chain. (See the demonstration specimen under the binoctdar 
microscope; or U. S. Agr. Dept. Bui. 120, fig. 2.) 

Make a diagramm.atic drawing to show the vegetative structures 
in position on the leaf -surf ace. 

The conidia are produced in great quantities. The}^ give the powder}'' 
appearance so characteristic of these mildews. They are scattered by the 
wind and initiate secondary cycles. 

After a period of conidial production, the mycelium begins to form the 
sexual fruit-bodies, — the perithecia. These usually begin to develop 
toward the end of the growing-season but before the leaves fall. In some 
cases as in the apple mildew-pathogene, P. leucotricha, the perithecia 
are formed on the twigs. The detailed study of the development of the 
sexual bodies and the formation of the perithecium cannot well be under- 
taken in this exercise. (See de Barv, Morphology and Biology of the 
Fungi, p. 226, fig. 107.) 



127 

Saprogenesis. The perithecia do not complete their development 
until the late autumn, usually after the leaves fall. Some do not mature 
until the following spring. They develop and mature at the expense 
of food gathered by the parasitic mycelium. Select from the material 
provided, representatives of two or more genera. Study the perithecia. 
observe: — 

31. The shape, color and structiue of the perithecium ; the 
nature of the appendages, draw one perithecium w4th its appendages. 
Study species from the three remaining genera, outline the perithecia, 
but DRAW carefully a typical appendage for each. (See Salmon, Mono- 
graph of the Erysiphaceae, pi. 1-7; also Burrill, Parasitic Fungi of 
Illinois, p. 395-397.) Crush the perithecium in each case and examine. 
In this connection the following key:— - 

A. Perithecia with one ascus. 

1. Appendages simple, undivided at tip sphaerotheca 

2. Appendages once or more dichotomously divided at the tip 

podosphaera 

B. Perithecia with several asci. 

1. Appendages never more than slightly swollen at the base. 

a. Appendages simple, or irregularly branched; without tip 

peculiarities erysiphe 

b. Appendages once or more dichotomously branched at the 

tip microsphaera 

c. Appendages spirally rolled at the tip uncinula 

2. Appendages swollen at the base so as to form an enlarged plate . . . 

PHYLLACTINIA 

With ascospore-discharge in the spring and early summer, saprogenesis 
of the primary cycle ends. 

The Secondary Cycles are, as pointed out above, initiated by the 
conidia from the primary lesions. They normally repeat in all details 
the phenomena and structures exhibited in primary cycles. Secondary 
cycles may repeatedly initiate other secondary cycles during the season. 

report 
1. Discuss control of one of the powdery mildews which may 
be selected, treating the subject under the headings of eradication and 
protection and explain how the life-habits of the pathogene make effective 
the measures described. 



APPLE SCAB 

Of all the diseases of the apple, this is the most common and best known 
to the growers. It is the one fungous disease for which they spray. It is 
world wide, occurring practically wherever the apple is grown. While 
there is a marked difference in the susceptibility of varieties, all will suffer 
some under conditions especially favorable to the fungus causing the 
disease. The scab of the pear is very similar in its symptoms to the apple 
scab but is caused by a distinct but closely related species of fungus. 

SYMPTOMS ' 

The disease affects the leaves, flowers, fruit and rarely the twigs. 
Material is provided showing the different symptoms. 

On the leaves. The first evidence of the disease in the spring is on the 
unfolding leaves. The scab-spots usually appear first on the lower surface, 
but later new spots appear on the upper surface. Examine the leaves 
provided and observe: — 

1. The size, form and character of the spot. The radiating char- 
acter of the lesion. To what due ? 

2. The character of the injury to the leaf. Does the injury show 
on the surface opposite the spot ? 

3. The difference in the character of the upper and under surface 
of the leaf itself; and of the scab-spots on the two surfaces of the leaf. 

4. The variations in the character of the scab-spots on differ- 
ent leaves. (Compare Cornell Bui. 335, pi. I.) 

Make drawings to show the characters of the scab-spots on the 
upper and under surfaces of the leaves. 

On the flowers. The disease may appear on the pedicle and calyx of 
the flower before the petals fall and may be severe enough to prevent the 
setting of the fruits. (See Cornell Bui. 335, pi. VII.) In the material 
provided, observe: — 

5. The location, form and character of the scab-spots and the 
effect on the flower. Make drawings to show these symptoms. 

On the fruit. Where the infection of the calyx is not severe enough to 
prevent the fruit from setting, the apple as it grows shows the enlarging 
scab-spots. These become very evident as the season advances. In the 
young apples provided, observe: — 

6. The black scab-spots. Their form, size, and effect on the 
fruit. To what region on the apple are they largely confined? 

7. The felty black center of the spot. In some cases, this felt 
has disappeared and the center of the spot is hard, reddish brown and 
often cracked. Note the scab-spots on the mature apple provided. 

8. The papery rim bordering the spot; best seen in the younger 
spots. This consists of the cuticle of the apple which has been loosened by 
the fungus as it spreads out from the center of the spot. (See Cornell Bui. 335. 
pl. VI, fig. 2.) 

Make drawings to show the points brought out in 6, 7 and 8. 
Sometimes these spots cause a dwarfing of the apple on the affected 
side. (See demonstration specimens.) 

128 



129 

On the twigs. This form of the disease appears to be rare except on 
certain varieties Hke the Lady apple. In Maine and other very northemly 
apple sections it is not uncommon on other varieties. In the material 
provided, observe: — 

9. The rough blistered character of the lesions, confined to the 
growth of the current year. draw. 

ETIOLOGY 

The apple scab is caused by the conidial stage, Fusicladium dendriticum 
(Wallroth) Fuckel,of an ascomycetous fungus known as Venturia incBqualis 
(Cooke) Winter ( = V. Pomi (Fries) Winter) . It belongs to that group of 
the ascomycetes known as Pyrenomycetes which have their asci enclosed 
in a more or less globose fruit-body, called a perithecium. 

Life-history. It is in the conidial stage that this fungus exhibits its 
parasitic nature. It lives superficially on the host or nearly so, simply 
prying off the cuticle or upper part of the epidermal cells, and applying 
its mycelium closely to the host-tissues. 

The Primary Cycle is initiated by ascospores from perithecia in old 
leaves on the ground. 

Pathogenesis. Crush in ]Dotassium hydroxide a bit of the old leaf 
provided, examine and observe : — 

10. The 2-celled olivaceous ascospores. draw. These ascos- 
pores are shot from the ascus which protrudes through the ostiolum of the 
perithecium. Ascospores are discharged only dtiring rains or very moist 
weather in spring. They are carried to the young leaves just emerging 
from the buds. Here they germinate. 

Study Cornell Bui. 335, pi. IX and X, and observe:— 

11. That but one cell of the ascospore gives rise to a germtube. 
Which cell? draw to show three stages in the development of the germ- 
tube. This germtube pierces the cuticle of the leaf or young fruit and initi- 
ates the scab-spot, copy Cornell Bui. 335, fig. 185. Again examine the 
scab-spots on the leaf and with the hand-lens or low-power, make out : — 

12. The radiating branched mycelial threads. Why do they 
radiate from a center? Aiake an enlarged drawing of a scab-spot to 
show this habit of the mycelium. 

Study the mycelium, as shown in the prepared slides of apple leaves 
which have been cooked in potassium hydroxide and the epidermis, bearing 
the fungus, peeled off. observe: — 

13. The form, size and septation of the mycelium. Its color and 
method of branching. No haustoria are sent into the host -cells and the 
mycelium does not at this stage penetrate beyond the epidennal cells. 

14. That the conidiopho -es arise in clusters or singly from this 
spreading mycelium. Note their form, length and color. 

15. The conidia lying about, which have been broken off from the 
conidiophores; their form, size and color and point where they were attached 
to the conidiophore. Several conidia may be produced from near the same 
point on a conidiophore. How? See if you can find a conidiophore which 
shows this. 

Make drawings to show the mycelium, conidiophores and conidia 
in their proper relations to each other and to the host. 



130 

Scrape some of the conidia from the scab-spot on an apple. Mount 
and study. Compare them with those found in the prepared shde. They 
are sometimes 1 -septate. 

Make drawings to show the variations in size, form and septation of 
the conidia. 

Sapr agenesis. The mycehum in the primary lesions on the leaves 
continues to produce conidia until the leaves fall. The fungus, which has 
up to that time lived practically on the surface, now sends new branches 
of the mycelium throughout the dead tissues of the leaf. From this 
mycelium are formed the globose perithecia. They are formed just 
beneath the epidermis of the lower or upper surface of the leaf, being 
most abundant near the surface facing upward as the leaf lies on the 
ground. Examine the old apple leaves provided and observe : — 

16. The evidence of the old scab-spots of the summer. 

17. With the hand-lens the pimple-like perithecia scattered over 
both surfaces of the leaf just beneath the epidermis. Note the color, size, 
distribution, relation to the old scab-spots, and relative abundance on 
two sides of the leaf. 

Make a drawing of a portion of the leaf to show the perithecia as 
seen with the hand-lens. 

With the scalpel, cut from the leaf a small square of tissue showing 
an abundance of the perithecia. Place between pieces of pith and with 
a razor make freehand sections through the perithecia. Mount and study. 
OBSERVE : — 

18. The shrunken dead condition of the host-tissue. 

19. The imbedded perithecia. 

20. The mycelium of the parasite throughout the tissues of the 
leaf; its form and character. 

21. The structure of the perithecium. (Best made out in the 
stained sections.) The walls, their relation to the host -tissues, form, size, 
mouth or ostiolium and asci, with ascospores. How many ascospores in 
an ascus? 

Make a drawing to show the structure and contents of the perithe- 
cium and its relation to the host-tissues. 

The perithecia begin to develop in the autumn but do not mature 
their ascospores until spring at the time when the apple leaves and blossoms 
are unfolding. 

Secondary Cycles are initiated by the conidia from the lesions on 
leaves and fruits on the tree. Aside from their conidial origin, the secondary 
cycles which are continuously initiated throughout the season, do not differ 
from the primar>^ 

22. Study the germinating conidia. draw. 

The fungus forms on the fruit a thick stroma-like growth from the 
upper surface of which arise the short conidiophores bearing the conidia. 
In order to study this structure and the relation of the parasite to the tissue 
of the fruit, make thin cross-sections through the scab-spots on the half- 
grown fruits, observe: — 

23. The thick stroma-like mass of the mycelium. Note its 
pseudoparenchymatous structure; thickest toward the center of the 
spot. 

24. The short conidiophores with conidia. Find conidiophores 
that show different stages in their formation and development. 



131. 

Make a drawing to show the structures observed. 

Pathological Histology. Study the prepared sections made through a 
scab-spot on the fruit, observe :— 

25. That the fungus worlcs in the cuticle above the epidermal cells, 
splitting off and forming the papery rim of cuticle that covers the advancing 
margin of the fungous growth. This is best made out at the very edge of 
the lesion. 

26. The suberization of the cells of the host just under the 
fungous stroma, indicated by the browning of the cell-walls. Why does 
this occur? 

REPORT 

1. Give a concise account of the life history of Venturia in- 
csqualis (Cke.) Wint. Arrange your data under and show the heads and 
subheads used in this outline. 



EMGOT OF RYE 

This disease is one of the earHest known and most frequently studied of 
ascomycetous diseases of cereals; due doubtless to the poisonous effects 
of the sclerotia of the pathogene on man and beast, rather than to the 
reduction of crop-yields which it causes. The host of greatest economic 
importance is rye. 

SYMPTOMS 

The heads alone show the disease which is exhibited chiefly by the 
presence of the pathogene structures. In the early stages, the pathogene 
produces a hone^^dew which exudes and nms down over the young spikelets. 
Examine the dried specimens labeled " honey dew-stage," and photograph 
1, fig. 2, 4 and 22, and photograph 9. observe: — 

1. Smudgy traces of the now dried honeydew. This honeydew 
is often difficult to see in dried specimens but in fresh specimens it is often 
very copious and sticky. 

Carefully dissect away the glumes in the smudgy region, observe: — 

2. The whitish kernels with corrugated surfaces. Compare 
with normal kernels from the same or healthy heads of the same age, 
as to size and color. 

3. The glumes about the diseased kernels. Are they diseased? 
Compare with healthy. 

It is from these diseased kernels that the honeydew oozes, but in the 
dried specimens, there is evident onl}^ the whitish covering of the dark 
diseased kernels. 

Make a drawing of the organs of a single diseased and healthy spikelet 
to bring out these characters. 

Later there is developed the ergot stage. Examine the specimens 
and photographs provided and observe: — 

4. The black spur-like bodies projecting here and there from 
between the glumes, — the ergots; average number from each head. Exam- 
ine illustration specimens of other ergot-infested grasses, draw a head 
of rye and a head of one other grass showing the ergots. 

Examine the loose ergots provided, note: — 

5. The variation in size and shape; consistency; the surface, 
often checked and cracked. These ergots are sclerotia of the pathogene 
developed, in place of the host-kernel, from the structures of the Sphacelia 
stage, the shrunken remnant of which may often be observed still clinging 
to the tip of the sclerotiimi. 

6. On breaking one open, the light colored interior. These 
ergots are poisonous. 

Make an enlarged detailed drawing of two or more of the ergots. 

ETIOLOGY 

The ergot is produced by the ascomycetous fungus, Claviceps purpurea 
(Fries) Tulasne, the conidial or honeydew-stage of which was given the 
name, Sphacelia segetum Leveille. 

Life-history. Several careful researches have resulted in a rather 
complete knowledge of the life-history of this pathogene. Primary and 
secondary cycles with sharply marked pathogenesis and saprogenesis 
occur. 

132 



133 

The Primary Cycles are initiated in early summer at the time the rye 
or other grass-hosts begin to head-out. 

Pathogenesis. Ascospores which are produced in the spring 
constitute the inoculum for the primary cycles. From the ergots (sclerotia), 
which overwinter on the ground, there develop in the spring, stalked 
fruit-bodies. In the heads of these are developed numerous perithecia 
containing the asci with long slender ascospores. Examine the specimens 
provided or photograph 4, fig. 22. observe : — 

7. The little pimple-like dots scattered thickly over the globose 
head of the fruit-body, — the ostiola or mouths of the perithecia. 

From these ostiola the long thread-like ascospores are shot into the air 
and, carried by air-currents, lodge in the infection-courts, — the open blos- 
soms of the rye or other grasses. 

A bit of material from a mature stroma will be provided (take clean 
slide to materials room). Crush, cover and study, observe: — 

8. The long slender ascospores; some floating free, others still 
in the asci; continuous or septate? draw several ascospores. (Keep this 
slide for No. 20.) 

These germinate within the blossom. Examine photograph or de Baiy, 
Morphology and Biology of the Fungi, p. 227, for genninating ascospores. 
NOTE : — 

9. That upon germination the spore at once becomes, to all 
intents, a part of the mycelium, copy. 

The branching germ tubes quickly penetrate the young ovary and, grow- 
ing throughout its tissues, develop the Sphacelia or conidial form of the 
fungus. 

Study cross-sections (freehand or prepared) of the upper end of the 
ovary (made during the Sphacelia stage). Examine with the high-power. 
observe : — 

10. The numerous irregular cavities lined with erect conidio- 
phores. Note the irregular dark masses of host-cells. 

draw a small portion of the h^^menium with underlying host-tissue, 
showing the conidia and the manner in which they are borne. 

The honeydew consists of a sweet fluid, exuded by the pathogene, 
in which great numbers of these conidia find their way to the surface of 
the spikelet. Insects attracted by the honeydew serve as inoculating agents 
in starting the secondary cycles. 

Very soon after, the conidia mature and the mycelium in the basal 
portion of the ovary begins to develop the sclerotium, which, as it grows 
and develops, replaces the host-cells and pushes forth as a long black spur 
with the shriveled Sphacelia structures at its tip. It is at first light 
colored, inclined to purplish but soon turns black. (See illustration 
specimen.) 

Examine photographs 3, fig. 25 and 4, fig. 13-14. note: — 

11. That while the sclerotium usually replaces the entire ovary, 
this is not always the case, copy from the above figures to show this. 

Make very thin cross-sections of one of the mature sclerotia and study 
under the high-power, observe: — 

12. The pseudoparenchymatous structures. 

13. The thick walls and small lumina of the hyphae composing 
the sclerotium. Are there any evidences of host-tissue? 



134 

14. The dark color of the outer coat or rind; number of cells 
thick; the white color of the medulla. 

Detail in a drawing the structure of the sclerotium. 

Saprogenesis. These sclerotia, when mature, fall to the ground or 
find their way at threshing into bins along with the rye grains. With 
these they may be sown and find their way to the soil. Here they remain 
dormant until spring, when the fungus again becomes active. At any 
point beneath the rind, growth-centers may be set up from which are 
developed slender stalks with globose tops. These are developed at the 
expense of the food-materials stored in the cell-walls of the sclerotial 
medulla. 

Examine the so-called germinating sclerotia provided, observe: — 

15. The smiall, capitate bodies arising from the sclerotium, — 
the stromata. Note the comparative size of the stem and the head. 
Examine the stromata carefully with a hand-lens, noting the small dots 
or punctations. Make a drawing to bring out all the external characters 
of the stromata and how they arise from the sclerotium. 

Prepared slides of cross-sections of the stromata are furnished. 
Examine with the low- and high -powers and observe : — 

1(3. The ovate acuminate cavities, — perithecia. Note the dense 
pseudoparenchymatous wall. 

17. The location of the perithecia about the periphery. 

18. The opening at the top, — ostiolum. Note that it pro- 
trudes. Make an outline drawing of the cross-section of the entire 
stroma. 

19. The asci arising from the bottom of the perithecium, 

20. Within the asci, the bundle of eight thread-like spores. 
Supplement at this point with the crushed mount prepared under number 8. 
Stain by running a little methyl blue under the cover-glass. 

Make an enlarged drawing of a perithecium containing the asci; 
and of a single enlarged ascus with the ascospores. 

Secondary Cycles initiated by the conidia of the Sphacelia stage, 
multiply and spread the pathogene during the blossoming-period of the 
host. The Sphacelia stage, followed by the development of the ergot, is 
produced as in the primary cycles. 

report 
1. Make one or more cartoon-like sketches to illustrate the life- 
history of Claviceps purpurea (Fr.) Tul. 



ONION SMUT 

This disease is most prevalent in the northeastern part of the United 
States where in certain localities it catises a considerable loss. Because 
of its striking symptonis it has been known to onion-growers for many- 
years and has been the subject of study by scientists for at least the past 
forty years. 

SYMPTOMS 

Examine a diseased seedling, observe: — 

1. The narrow, yellow lesions extending pa,rallel with the leaves. 
These are the first symptoms to appear. 

2. The narrow, black areas which are most numerous near 
the base of the onion and sometimics extend almost to the tip of the leaf. 
The yellow areas will later become darkened like these. 

3. That these black masses are all enclosed within the tissue of 
the leaf, or that occasionally the epidermiis is ruptured. 

4. That in cases where the leaf is badly affected, its tip has 
withered and droops. 

Make a drawing of a diseased seedling. 

Examine one of the larger diseased onions, observe : — 

5. That in this case each badly diseased leaf has turned brown 
and has fallen over. 

6. That the black lesions have now broken open, forming long 
black oi^en sori from which a sooty-like mass is sifting, draw. 

ETIOLOGY 

The pathogene reponsible for this disease is Urocystis Cepulae Frost, 
a basidiomycetous fungus of the order Ustilaginales. There are two 
families in this order, the Ustilaginaceae and Tilletiaceae. To the former 
belong those organism.s which produce the loose smiUts of wheat, oats 
and corn, while to the latter belong the pathogenes like those producing 
the stinking sm^ut of wheat, and the onion smut. 

Life-history. 

The Primary Cycle has its origin in the chlamydospores found in 
the infested soil. 

Pathogenesis. These spores, when near germinating onion seeds, 
also germinate sending out a long branched m\^celial thread. This my- 
celium probably enters the host directly, or it may possibly produce conidia 
or sporidia borne on the tips of the mycelial branches. In the latter case 
it is the germtube which grows from the sporidimii, that enters the tissue of 
the host. Infection alwa^'s takes place imder ground and only when the 
seedling is just emerging from the soil. No new infections occur after the 
onion becomes older. This mycelium develops within the leaf-tissues, 
giving rise to the long black sori. Scrape a few spore-balls from a sorus 
and, under the microscope, observe: — 

7. The size, color and surface markings of the spore-ball. It is 
made up of a central thick-walled re sting-spore (one or more celled) 
surrounded completely by thin-walled pseudospores. Only the central 
cells are viable. If the inner structure can not be determined from the 
mount, consult Duggar, Fungous Diseases of Plants, p. 382. 

135 



130 

Make a diagrammatic drawing showing the structure of the spore-ball. 

If no germinating spores are available, sketch the illustration shown in 
the above reference. 

Cut a cross-section of an onion seedling through a closed lesion or use 
prepared slides, observe: — 

8. The depth to which the sorus penetrates. 

9. The black mass of spores lining the sorus. Are they attached 
to stalks? How are the spores of a smut-pathogene usually borne? (See 
McAlpine, Smuts of Australia, p. 19.) 

10. The cuticle still intact and retaining the spores which later 
escape. 

11. The mycelium penetrating the tissues adjoining the sorus. 
Do any of these cells appear dead? 

Make a drawing of a cross-section of a sorus with the host-tissue 
surrounding it. 

Sapr agenesis. There is no vegetative activity during saprogenesis. 
The spores lie dorrfiant in the soil or host-debris. They have been known 
to live over in the soil as long as twelve years. 

Secondary cycle. Unlike most fungi this pathogene has no secondary 
cycle. Infection takes place only during a limited period while the host 
is in the seedling stage. They are initiated only by overwintered spores. 
Consequently, even if onion sets are transplanted to badly infested soil, 
the onions will remain free from the disease. 

REPORT 

1. Discuss the use of sulphur and of formaldehyde in the control 
of this disease, including the advantages or disadvantages in methods of 
application, the amounts used, cost, and the preference of the onion- 
grower of the present day. 



LOOSE SMUT OF WHEAT 

This disease is exceedingly common and often destructive in the wheat- 
fields of eastern United States and Canada. It is readily distinguished 
from the stinking smut. The latter, while not uncommon in the east, 
is much more general and destructive than loose smut in the great wheat- 
lands of the west. While the loose smuts of wheat and oats are much alike 
in general appearance, they differ strikingly in certain features of the life- 
history of the respective pathogenes. 

SYMPTOMS 
The evidences of the disease are to be observed chiefly in the heads at 
blossomiing-time. Compare the diseased and healthy heads provided. 
NOTE : — 

1. The general effect of the disease on the form and appearance 
of the head. Does it affect the rachis as to length and size; length of 
intemodesr' draw both diseased and healthy heads. 

2. The effect on the individual s pikelets. Determine this by 
carefully dissecting out the parts of the (moistened) healthy spikelets 
and flowers, draw. Dissect and draw the parts of the diseased spikelet. 

3. That the culms of a diseased plant at first grow more rapidly 
and outstrip those of the healthy plant. Eventually, after spore dispersal, 
the healthy culms push up above the diseased ones. (See whether later 
studies will explain this.) 

4. The naked ra chides from which the spore-m.asses have 
disappeared, draw. 

ETIOLOGY 

The loose smut of wheat is caused by the basidiomycetous parasite, 
Ustilago Tritici (Persoon) Jensen. It is a member of the order of primitive 
basidiomycetes, the Ustilaginales commonly known as the smut-fungi. 
There are two families in this order, the Ustilaginaceae to which U. Tritici 
belongs and the Tilletiaceae to which belongs the stinking smut -pathogenes, 
Tilletia Tritici (Bjerk.) Wint. and Tilletia foetens (B. and C.) Trel. 

Life-history. While the parasite causing this disease has long been 
known, it is but recently that its life-history has been completely under- 
stood. (See PI. Ind. Bur. Bui. 152:10-12.) It differs, along with U. nuda 
(Jens.) Kell. and Sw. in barley, from most of the other smut-fungi whose 
life-histor}' is known, in that infection occurs through the stigmatic sur- 
faces of the pistil at flowering-time. There are no secondary cycles. 

The Primary Cycle requires a full year for its completion. There is 
no saprogenesis, the pathogene remaining in continuous association with 
the living host except for the few minutes during transfer from the source 
of inoculiim to the infection-court. 

Pathogenesis. The black smutted heads emerging from the upper 
leaf -sheaths at flowering-time constitute the sources of inoculum. 

Mount some of the soot}^ mass in a drop of potassium hydroxide, cover 
and examine with the high-power, observe: — 

5. The numerous brown globose bodies scattered through the 
mount, — the chlamydospores. These, in the case of this smut-fungus, 
constitute the inoculum. 

137 



138 

6. Their color (lighter on one side); markings; variations in 
form. (See demonstration microscope.) draw five different spores. 

While the chlam3^dospores of most smut-fungi are resting-spores, 
those of U. Tritici, on account of the blossom-infecting habit of the patho- 
gene, have largely lost their ability to remain viable for a long period. 

The dusty dry chlamydospores are scattered by the wind over the 
blossoming wheat. Some spores fall upon the exposed stigmas protruding 
from the open glumes of healthy heads. Here they germinate, sending 
a long germtube along and into the stigmatic filaments and by way of 
them into the young ovary. If germinating spores are available, study and 
observe;- - 

7. The long genntubes, septate and with knee-joint fusions. 
S. The uniform absence of sporidia. 

DRAW several germinating chlamydospores to show variation. 

This germtube is morphologically a basidium which in most other 
species of Ustilago produces basidiospores or sporidia as they are called. 
In this species, sporidial production is unnecessary. After penetrating 
to the ovary, the germtube branches and, establishing itself near the grow- 
ing-point, goes into a dormant condition along with the ripening kernel. 
The invaded kernel shows no evidence of injury. 

As the embryonic plant develops upon planting the kernel, the mycelium, 
aroused by the same condition of heat and moisture (and probably by the 
activities of the embryo as well), begins active growth, branches and spreads 
into the stooling culms, and forces its way upward through the tissue 
as the stem lengthens. When the wheat heads begin to form, the mycelium 
of the fungus takes possession, completely replacing the more tender 
or succulent tissues of the spikelet with its dense mats of profusely branch- 
ing mycelium, thus forming the black, easily ruptured sori of the smut. 
The diseased heads emerge from the sheath about the time the flowers 
on the healthy heads are in full bloom. Why? 

REPORT 

1. A farmer writes for information on the nature and control 
of the loose smut of wheat. Write him a letter giving clearly and concisely 
the information he needs. 



LOOSE SMUT OF OATS 

There are two smuts of oats, the loose and the covered. Loose sinut 
is much the more common and the one usually referred to as oat smut. 
This disease occurs wherever oats are grown and may often destroy from 
25 to 50 percent of the crop. Its control is exceedingly simple and effective. 

vSYMPTOMS 

The evidences of loose smut are usually confined to the inflorescence, 
though the leaves may rarely show lesions. Compare the specimens of 
this disease with healthy specimens provided, observe : — 

1. The striking difference in the panicles of the two; formi, size 
and color, draw. 

2. In the entire plants (illustration specimens) the relative 
length of straw, number of stalks in the stool, amount and character of 
leafage. 

3. The differences in the spikelets of each; empty glumes, hull 
and flower parts (most easily determined by dissecting specimens in water) . 
Make enlarged drawings to show comparatively the effects of the dis- 
ease on the parts of the spikelet. 

Compare specimens of loose smut with those of covered smut, sketch 
to show differences. 

ETIOLOGY 

The pathogene causing the loose smut of oats is Ustilago Avenae 
(Persoon) Jensen. It is closely related to Ustilago Tritici (Pers.) Jens, 
but differs strikingly in certain features of it slife-history. (See p. 137) 
The covered smut of oats, caused b^^ Ustilago levis (Kell. and Sw.) Mag., 
occurs along with U . Avenae from which it is to be distinguished certainly 
only by its sm^ooth, granular spores. 

Life-history. There are in the life-history of this pathogene only 
primary cycles. While not so intimately associated with the living host 
during the resting-period as is Ustilago Tritici (Pers.) Jens., it normally 
exhibits little saprogenic activity under natural conditions. 

The Primary Cycles are initiated at blossoming-time, at which time 
inoculation occurs. 

Pathogenesis. The smutted plants scattered through the field 
are the sources of inocidum. 

Remove a bit of the black mass from a smutted head to a drop of 
potassium hydroxide on a slide; cover and examine, observe: — 

4. The numerous brown, globose bodies scattered through 
the mount, — the chlam^^dospores. 

5. Their color (lighter on one side) ; markings and variations 
in size, drav.' three different spores. (See demonstration microscope, 
oil-immersion.) 

These chlamydospores are resting-spores and may retain their vitality 
for several years. 

The chlamydospores, disseminated when the oats are in blossom, 
lodge within the oat hull next to the kernel. They do not geruxinate 
at once as do those of U. Tritici but lie dormant as the oat hull closes 
about the maturing kernel. The chlamydospores are thus enclosed 

139 



140 

along with the ripe kernel. When the oat grain is planted in the soil 
and germinates, the same conditions that cause the seed to grow, start 
the spores into activity. 

Spores have been germinated on the sHdes provided. Carefully cover 
the drop of water containing the germinated spores, and examine, ob- 
serve : — 

6. The rather long hyaline germtube or promycelium, — the 
basidiimi of this basidiomycete. 

7. That it is septate; number of cells into which it is divided; 
the crack in the chlamydospore-wall where the promycelium emerges. 

8. That from each cell at the septum (above or below?) arises 
one or niore oblong sporidia, — the basidiospores. 

9. The variations from the normal germination. 

Make drawings to show germination of chlamydospores and formation 
of sporidia, with such variations as are observed. 

10. Examine the mount of germinated spores to see if you 
can find any of the sporidia germinating, draw. 

As the seedling bursts through the seed-coat and pushes up through the 
hull, it is penetrated by the germtubes of the sporidia. The mycelium 
of the parasite grows and branches, pushing its way upward between the 
cells of the host as the culm.s of the oat come out and shoot upward. 
Usually every culm in the stool is infested. The mycelium at first stimu- 
lates the growth of the plant. The activities of the mycelium continue 
throughout the growth and development of the host until the formation 
of the flowers begins. In these rapidly developing embryonic tissues, 
the mycelium takes possession, appropriates the abundant food -substances 
and begins the formation of chlamydospores. The formation of these 
chlamydospores takes place as follows: — The m^^celium in the flower 
at the time that spore-formation begins shows a nodulate appearance 
and the branches are closely fasicled like clusters of grapes. Within each 
swollen part of the mycelium a chlamydospore is formed. As the chla- 
mydospores mature, the encompassing walls of the parent-hyphae and 
much" of the general m.yceliimi which is not differentiated into spores, 
gelatinizes or otherwise breaks away and the spores are set free in large 
masses. 

Practically all the floral parts of every spikelet are destroyed, trans- 
forming them into the black sori of the pathogene. The smutted panicles 
emerge, the spore-masses ripen, and the spores are scattered when the 
healthy panicles are in full bloomi. 

report 
1. Write a concise explanation of why the formaldehyde- 
treatment succeeds with loose smut of oats and fails with loose smut of 
wheat. 



STINUNG SMUT OF WHEAT 

Although the loose smut is the more common on wheat in eastern 
United States, the stinking smut is frequently serious. In the great wheat- 
fields of the west it is the common and destructive wheat smut. The 
losses from the stinking smut may be very large, not only from the reduc- 
tion in yield but also from the lowering of the market-value of grain with 
which the smutted kernels become mixed. The life-history of the causal 
fungus is ver\^ different from that inducing the loose smut of wheat. 

SYMPTOMS 

Heads of wheat affected with the stinking smut are readily distinguished 
from the healthy heads as soon as they emerge from the leaf-sheath. 
Read Barrus, Phytopath. 6:21-2S; study fig. 2; and the diseased and 
healthy specimens collected in different stages of development. 
NOTE : — 

1. The difference in size and color of the heads. 

2. On dissecting flowers, the differences exhibited by ovaries 
and stamens; size, color, form and odor. 

Confirm, so far as available material will permit, the obser^^ations 
of Barrus. Make a series of drawings to show the comparative morphol- 
ogy of diseased and healthy heads and flowers, from flowering-time to 
maturity. 

ETIOLOGY 

This disease is caused b}^ Tilletia Tritici (Bierkander) Winter, or by 
Tilletia joeiens (Berkley and Curtis) Trelease, { = T . laevi s Yiixhn) . This 
is a case in which two distinctly different species of fungi cause the same 
type of disease, in fact the two species may occur together in the same 
ovar\'. It is the latter form, T. foetens, which is most commonly found 
in eastern United States and will be the one here considered. These are 
species of the Ustilaginales, family Tilletiaceae. 

Life-history. This pathogene exhibits only the primary c^^cles in its 
life-history. Since the chlamydospores usually become attached to 
the health}^ grains during harv^esting or threshing and so are almost con- 
tinuously associated with the living host, this smut-fungus like most others 
may be said to have normalh' no saprogenic phase. Infection from spores 
in the soil appears to be rare for this species. The sown wheat in the 
Pacific Coast regions is said to become inoculated by wind-borne spores of 
T. Tritici. 

Pathogenesis. Inoculation, as indicated above, occurs at thresh- 
ing-time, when spores from the crushed smutted kernels become attached 
to the healthy grains. Crush one of the diseased grains between thumb 
and finger, note: — 

3. The greasy character of the spore-mass. This evidently 
helps to attach the spores to the seed-coat. 

Mount a bit of the spore-mass in a drop of water. Cover and examine. 
OBSERVE : — 

4. The numerous, more or less globose, sinooth brown spores, 
— the chlamydospores. draw. 

141 



142 

5. The chlamydospores of Tilletia Tritici under the demonstra- 
tion microscope. Note the reticulated surface of the spores, the globose 
shape and smaller size. ' draw. 

Make a mount containing chlamydospores of both Ustilago Tritici 
and Tilletia foetens. Compare as to size, shape, color and markings. 
DRAAV to show the contrast. 

When the inoculated grains are planted, the spores of the pathogene 
are planted with them. The conditions which favor seed-germination 
also favor spore-germination. A promycelinm is foiTned bearing sporidia 
which in turn produce germtubes capable of penetrating the seedling, 
but only at some point below the first node or tillering-point. Study the 
germinated spores provided, or figures in Washington Bui. 126:7, and 
OBSERVE : — 

6. The germtube arising from the chlamydospores, — the promy- 
celinm. From which side of the spore does it arise? 

7. The cluster of sickle-shaped sporidia at the end of the promy- 
celium. 

S. That frequently two of the sporidia have joined by a short 
tube forming an H-like figure. This is very common and was once sup- 
posed to be a sexual process but it is not now so considered. 

9. Some of the sporidia germinated, forming secondary sporidia. 

Make drawings to show chlamydospore-germination and formation 
of secondary sporidia, or copy from Minnesota Bui. 133, pi. XXIV and 
XXV. 

The germtubes from sporidia or secondary sporidia penetrate the seed- 
ling as it emicrges from the grain and, branching, forms a mycelium. This 
mycelium quickly reaches the growing-point and grows upward with the 
c>3lm causing no apparent injury to the host. At the time of flower-forma- 
tion, however, the hyphae develop rapidly and, growing into the ovary, 
destroy the contents and replace them with the black spore-mass. 

REPORT 

1. A farmer who knows how to control stinking smut desires 
to understand the cause of the disease and just how it differs in this respect 
from the loose smut of wheat. Write him a letter giving this information 
in a form which he will understand. 



ASPARAGUS MUST 

This disease is of European origin, and first appeared in America in 
several localities along the Atlantic seaboard about 1S9G. It rapidly- 
appeared farther and farther west until by 1902 it had become an important 
factor in asparagus-growing in California. It is much more destructive 
in- this countr}^ than in Europe. It is the most important disease of 
asparagus in America. 

SYMPTOMS 

Most rust diseases exhibit themselves in some form of hypoplastic 
or metaplastic effect. While the very first effect of the asparagus rust is 
probably hypoplastic in character, this rapidly passes over into a necrotic 
condition, the host-tissues being rather quickly killed. The signs and 
symptoms of rust diseases group themselves about certain distinct spore- 
producing stages of the pathogene. In the case of the asparagus rust there 
are three: the cluster-cup stage, the red rust stage and the black rust stage. 
All of these occur on asparagus. 

The cluster-cup stage. The disease in this stage affects oiily the stalks 
in the young condition shortly after they come up in the early spring. 
Examine the specimens provided and observe: — 

1. The oval-shaped, light-green patches on the canes near 
the base, covered with minute honey-colored pimples. These are the 
spermagonial fruit-bodies of the pathogene, — the pycnia. 

2. On the lesions as they grow older, larger pustules arranged 
in concentric order within the lesions, especially definite in the older spots; 
averaging how many to a lesion ? 

3. That finally these pustules break through the epidermis 
and appear as ro\md cup-shaped bodies, — the aecia, exposing the yellow 
spore-mass within. 

This phase of the disease is usually to be found only on volunteer plants 
along roadsides and hedgerows, on plants in abandoned plantations 
and in young uncut plantations. Make a drawing to show the lesion 
of the cluster-cup stage. 

The red rust stage. This usually appears early in the summer on the 
plants which are allowed to grow up after the cutting-season. In the 
specimens provided, observe: — 

4. The rusty brown linear pustules scattered very abundantly 
over the stem, branches and even on the needles. — the uredinial sori or 
uredinia. 

5. Their size, n-umber and arrangement on the surface; relation 
to the host-tissues. 

G. The dusty character of their contents. 

7. Any pathogenic effects exhibited by the tissues about the 
sori. 

Make a drawing of a diseased stem; also a drawing of a uredinium 
as seen under the hand-lens. 

The black rust stage. This phase of the disease begins to develop 
in late summ^er on the same branches and needles along with the red rust 
stage. During the transition, affected plants take on a brown color 
which, however, soon gives place to a distinct black, as uredinial develop- 

143 



144 

ment ceases and the black resting-spores of the pathogene predominate. 
Examine the old diseased branches and needles provided and observe: — 
S. The crowded black sori or telia. How do they compare 
with the uredinia as to prominence, shape, size, arrangement on the stems 
and branches, and dustiness of their spore-masses? 

9. That the host -tissues covered with the telia are dead and 
shriveled. This is the last phase of the disease, the autumn or winter 
condition of rusted plants. 

Make a drawing of a diseased stem, and also of the pustules as seen 
under the hand-lens. 

ETIOLOGY 

The asparagus rust is caused by Puccinia Asparagi DeCandolle, a 
species of the Uredinales, an order of fungi, the members of which are the 
most highly specialized of the Basidiomycetes. It is an autoecious patho- 
gene producing its different spore-forms all upon the same host. 

Life-history. It produces during its life-cycles all the known fruiting 
structures of the rusts: pycnia (O), aecia (I), uredinia (II), telia (III), 
and basidia (IV), each with their characteristic spores. It is therefore said 
to be a Eupuccinia, that is, a true Puccinia. (See McAlpine, Rusts of 
Australia, p. 10-11.) 

The Primary Cycles have their development early in the spring on 
the young shoots before July first; often as early as March but usually 
during April and May, depending upon the locality and season. Pycnia 
and aecia only are developed during the primary cycles. 

Pathogenesis. The inoctilum for the primary infections consists 
of the basidiospores or sporidia produced on the promycelium from the 
overwintered teliospores on diseased stalks and needles. Examine the 
germinated teliospores on slides provided; or study California Bui. 165, 
fig. 27. observe: — 

10. The long slender promycelium (basidium) put forth from 
each cell of the teliospore; 4-celled near the apex. 

11. The long pointed sterigma from each basidial cell, bearing 
at its apex a thin-walled ovoidal basidiospore. Each spore contains one 
nucleus and finely granular protoplasm.. 

DRAW or COPY to show sporidia ready for dissemination. 

These sporidia are carried by the wind or splashing rain to nearby 
shoots just coming up. There in the moisture on the surface they send 
forth a germtube which penetrates the tender epidermis and gives rise to 
a locally spreading mycelium.. From the mycelium, within less than a 
month, are produced first pycnia and promptly after them, aecia. 

Study cross-sections of the stalk (freehand or prepared) through an 
aecial lesion; or California Bui. 165, fig. 18. observe: — 

12. The large cup-like aecia and scattered amoiig them the 
minute flask-shaped pycnia (spermogonia) . 

Locate a pycnium and study its structure under the high-power, make 
OUT : — 

13. The very slender interwoven mycelium forming the pycnial 
wall. 

14. The long slender sporophores arising from the wall-mycelium 
and converging at the center of the pycnium about a small cavity which 
is filled with the very minute bacterium-like pycnospores (spermatia). 



145 

15. The protruding neck of the ostiolum through which the 
spermatia are discharged. These pycnospores have now no known 
function. They will not germinate and are supposed to be vestigial male 
cells (sperms) . 

Study the aecia and observe: — 

16. That they are sunken in the tissues; at first covered by 
the epidermis of the host. Locate one that has not burst through the 
epidermis. The pycnia are more superficial; subepidermal or sub- 
cuticular? 

17. The fine interwoven mycelial threads between the host- 
cells about the aecia. Do they ever penetrate into the host-cells? 

IS. That it is from a densely interwoven mat of this mycelium 
that the structures of the aecium are developed; to be seen at its base. 

19. The closely packed club-shaped sporophores in the base 
of the cup, arising from the mycelial mat. 

20. The chains of spores cut off from each sporophore, forming 
the parenchyma-like mass that fills the cup. Why are the immature 
aeciospores angular in outline? Note that the maturing spores at the 
mouth of the aecium become globose. 

21. The large cells forming a lining to the cup, — peridial cells. 
Thc}^ also arise in chains from short mycelial branches similar to the 
sporophores. Note the difference in the thickness of the wall of the inner 
and outer faces of the cells. This lining of the cup is pseudoparenchyma- 
tous and is called the peridium. 

The mycelium of the aecial stage arises from the one-celled basidiospore, 
and each cell is uninucleate. When the formation of the aecivim^ begins, 
there is a sexual fusion of many pairs of mycelial branches in the region 
of what is to be the base of the cup. From each pair of branches arises 
a single binucleated stalk, — a sporophore of the aecium. A chain of 
spores are developed from each of these binucleate sporophores by suc- 
cessive conjugate nuclear divisions and a laying-down of a septum between 
the pairs of nuclei thus formed. Examine stained sections carefully under 
high-power and locate : — 

22. The paired nuclei in the sporophores; in the aeciospores. 
Make a drawing to show the structure of the aecium, the pycnium 

and their relation to the host -tissues as seen in longisection. 

Remove with a needle or scalpel some mature aeciospores from the 
diseased stalks provided. If dry, mount in potassium hydroxide. Study 
and OBSERVE : — 

23. The form, size, color, contents and markings on the epispore. 
DRAW several aeciospores. 

But one crop of aeciospores is produced. With the maturity and 
discharge of the last of them the activities of the primary cycle cease. 
The aeciospores initiate secondary cycles through which alone the patho- 
gene perpetuates itself until the following year. There is no saprogenesis 
in the primary cycle. 

Secondary Cycles develop on the stems, branches and needles of 
stalks affected by the aecial stage and on those of neighboring asparagus 
plants. 

Pathogenesis. The first secondary cycles are initiated by aecio- 
spores which fall upon the host. These germinate in the dew which 
forms at night and send out each a germtube which rapidly grows along 



146 

over the surface until its tip reaches a stoma through which it penetrates 
to the tissue within. The protoplasm of the spore passes out into the germ- 
tube keeping constantly in its tip as it grows. Within the substomatal 
cavity, the tip of the germtube applies itself to one of the parenchymal 
cells and sends into it a short blunt branch, — a haustorium. Having'thus 
established a food relation with the host, it grows and develops a ramifying 
binucleate mycelivmi. This mycelium spreads only locally and produces 
within two or three weeks mature uredinia. Scrape some uredospores from 
a diseased stalk, mount in potassium hydroxide and observe . — 

24. The form, size, color and contents of the uredospores as 
compared with the aeciospores; markings on the epispore if any. draw 
several aeciospores. 

Study cross-sections (freehand or prepared) of a stem thror.gh a ured- 
inium and make out: — 

25. The structure of the sorus and its relation to the host-tissue. 

26. The intercelhtlar mycelium. Are haustoria formed? (See 
California Bui. 165, fig. 16.) 

27. The stout sporophores arising from the mycelium, each bear- 
ing at its apex a uredospore in some stage of developmicnt or mature. 
How can one tell whether the spore is young or mature? 

Make a drawing of a part of the cross-section through the uredinium. 

These uredospores are usually wind-disseminated and initiate other 
secondar}" cycles. They germinate at once in moisture, even in a moist 
atmosphere. They infect the host in a manner and under conditions simi- 
lar to the aeciospores. The germtubes emerge through special pores or 
thin places in the spore-wall. Mount and examine uredospores that have 
been treated with acetic acid, observe: — 

28. The size, number and location of the pores, draw. For 
germination and penetration into the host, study California Bui. 165, 
fig. 21-22. Supplement with study of germinating spores on slides, if 
available. Make drawings to show the points brought out. 

From the mycelium produced from the uredospore, other uredinia 
with mature uredospores may be produced in twelve days under favorable 
conditions. This is repeated over and over throughout the growing-season. 
The mycelimn remains binticleate throughout all the uredospore- 
generations, the spores being binucleate also. 

As the host begins to mature, there is developed from the mycelium 
in the secondar}^ lesions, not uredospores but another spore-form, — 
chlamydospores, l^lack resting-spores known as teliospores (teleutospores) . 
They often arise at first in the same sorus with the uredospores, later they 
appear in sori in which no uredospores are formed. Mount some telio- 
spores in potassium hydroxide and study: — 

29. Their form, size and structure as compared with the uredo- 
spores; color, where located, in walls or contents? 

30. The thick walls and densely granular oily contents. 

31. The light spot in the center of each cell; not always evident. 
The nuclei are located here. These spores are at first binucleate but at, or 
just before the germination the nuclei fuse and sexual fertilization, begun 
by the association of nuclei in the aecium, is now completed. 

The structure of the telium and its relation to the host-tissues does not 
differ materially from that of the uredinium. draw several teliospores 
to show structures and variations in form and size observed. 



147 

Sapr agenesis . With the formation of the teHospores, the host 
has succumbed. The rusted twigs and needles fall to the groutid while 
the old stalks stand erect during the winter or fall under the weight of snow. 
On these stalks, twigs and needles, the pathogene in the fonn of domiant 
teliospores passes the winter. Under the conditions of moisture and 
temperature that start the host into growth in the spring, the teliospores 
on the old host-debris become active. They germinate in situ forming 
a promycelium from each cell of the spore. Four basidiospores are pro- 
duced on each basidium. Each is uninucleate, its nucleus having been 
derived through the reduction division of the fusion nucleus of the telio- 
spore. These sporidia, as we have seen, constitute the primary inoculum 
which infects the young asparagus shoots and produces the aecia. 

REPORT 

1. Prepare a diagram showing the spore-forms, and the activi- 
ties of P. Asparagi DC. in proper sequence through the primary and 
secondary cycles. 

2. Give in some detail three fundamentally different methods of 
controlling the rust of asparagus, arranging them in order of practicability 
and efficiency. Justify by explanation and argument, the position to which 
•each method is assigned in the sequence. 



BLACK STEM=RUST OF CEREALS AND GRASSES 

There are many rust diseases of cereals and grasses. The black stem- 
rust is perhaps of greater economic importance than any of the others, 
especially on wheat. It is one of the earliest-recorded rust diseases and 
the subject of many researches by mycologists and plant pathologists. 
Early in the spring it may affect the barberry {Berberis vulgaris L.). 
At the same time or later in the season it appears on wheat, oats, barley, 
rye and many other grasses. On cereals and grasses it is characteristically 
a stem -rust. 

SYMPTOMS 

This disease is far more injurious to cereals and grasses than to the bar- 
berry. The signs and symptoms exhibited by rust diseases are associated 
with certain distinct spore-producing stages of the causal fungus. In the 
case of the black rust of cereals there are three : the cluster-cup stage, the 
red rust stage and the black rust stage. 

Cluster-cup stage. The leaves and flower-racemes of the barberry are 
the organs affected. Examine the leaves provided and observe : — 

1. The small spot-lesions; size, location, color both above and 
below, and the thickening of the leaf-tissues in the lesion. The disease 
exhibits distinct metaplastic effects on this host; there is a slight over- 
growth of affected tissues. 

2. With the hand-lens, the tiny dark amber or nearly black 
pimples embedded in the tissues of the upper surface, — the pycnia (sper- 
magonia), functionless structures of the pathogene. They normally 
accompany or precede, in the same lesion, the aecial fruit-bodies next to 
be observed. 

3. On the lower surface, the sunken cups, exposing their yellow 
spore-masses. These are the aecia or cluster-cups. 

Make drawings to show the cluster-cup symptoms on both surfaces 
of the leaf. 

Examine the diseased flower-clusters, observe: — 

4. The swollen and malformed flowers and pedicels; color of 
the affected organs. 

5. The abundantly developed aecia. Are pycnia present? 
Sketch a diseased flower cluster. 

Red rust stage. Examine the specimens of diseased wheat or oats 
provided and observe: — 

6. The large reddish yellow pustules scattered over the leaf- 
sheaths, — the uredinia; not usually so common nor so numerous on the 
blades. 

7. The size, arrangement and relation of these pustules to the 
host-tissues. Note the torn edges of the epidermal covering and the dusty 
spore-mass of the ruptured uredinia. 

8. The effect on the host-tissues; color effects. 

sketch a rusted leaf-sheath to show the symptoms of the red rust stage. 
Make a drawing of one of the uredinia as it appears under the hand-lens. 

Black rust stage. This develops during the latter part of the growing- 
season along with the red rust stage so that the two are often observed 
together on the leaf-sheaths and stems. In the material provided, 
observe : — 

148 



149 

9. The large black linear sori extending up and down the stem, — • 
the telia. Most of them have ruptured the epidermis, exposing the black 
spore-mass within. 

10. The thin epidermal covering, ruptured in most of the telia 
but partially or wholly covering others. 

11. The telia developed on the inflorescence. What organs 
are aflEected? 

12. Kernels from badly diseased pAants in comparison with 
those from healthy plants. How do they appear to be affected? Telial 
sori are occasionally developed under the seed-coat of the kernel. (See 
Phytopath. 1:150-154, fig. 1-2.) 

SKETCH an intemode and also parts of the inflorescence to show the 
black rust symptoms. Make an enlarged drawing of a telimn as seen 
under the hand-lens. 

ETIOLOGY 

The black rust of cereals and grasses is caused by the fungus, Puccinia 
graminis Persoon. It is one of the Uredinales which are perhaps the most 
highly specialized of basidiomycetous pathogenes. It is a heteroecious 
parasite producing its aecial structures on the barberry, a host widely 
remote in relationship from its grass-hosts on which the other spore- 
forms appear. This species, Puccinia graminis, includes a variety of 
biologic forms, morphologically alike, and all having their aecia on the 
barberry but restricted rather sharply in their uredinial and telial stages 
to definite cereal- or grass-hosts. (See McAlpine, Rusts of Australia, 

P- 122.) _ 

Life-history. This pathogene produces, during its life-history, all the 
known fi-uiting structures of the rust -fungi: pycnia (O), aecia (I), uredinia 
(II), telia (Hi), and basidia (IV), each with their characteristic spores. 
It is therefore said to be a Eupuccinia, that is, a true Puccinia. (See 
McAlpine, Rusts of AustraHa, p. 10-11.) 

The Primary Cycles are normally those which develop only upon the 
barberry and exhibit the pycnia and aecia. It seems certain, however, 
that the fungus may winter over in the uredinial form (probably as my- 
celium) and by uredospores initiate primary infections on grass- and 
cereal -hosts. This second type of primary cycle is essentially secondary 
in all respects except as to the time it originates. It wiU not be considered 
separatel}^ in this exercise. 

Pathogenesis. The inoculum for the primary infections consists 
of the basidiospores (sporidia), produced on the basidium (promycelium) 
from the overwintered teliospores on diseased straw. Examine the 
germinated teliospores on slides or study the drawings provided, and 
OBSERVE : — 

13. The long slender basidium put forth from each cell of the 
teliospore; 4-celled near the apex. 

14. The long pointed sterigma from each basidial cell bearing at 
its apex a thin-walled ovoidal basidiospore (sporidium). Each sporidium 
contains one nucleus and finely granular protoplasm. 

DRAW or COPY to show sporidia ready for dissemination. 

These sporidia are carried by the wind or splashing rain to nearby bar- 
berry bushes where, in the moisture on the surface, they send forth a germ- 
tube which penetrates the tender epidermis and gives rise to the locally 



150 

spreading mycelium. From the mycelium, within less than a month, 
are produced first yjycnia and promptly after them, aecia. 

Study cross-sections of the leaf (freehand or prepared) through an 
aecial lesion, observe: — 

15. The large cup-like aecia and opposite them on the upper 
surface of the leaf, the minute flask-shaped pycnia (spermagonia). 

Locate a pycnium and study its structure under high-power, make 
OUT : — 

16. The very slender intenvoven mycelium forming the pycnial 
wall. 

17. The long slender sporophores arising from the wall-mycelium 
and converging at the center of the pycnium about a small cavity which 
is filled with very mintite bacteri-um-like pycnospores (spermatia). 

IS. The protruding neck of the ostiolum through which the 
spermatia are discharged. These spermatia have now no known function. 
They will not germinate and are supposed to be vestigial male cells (sperms). 
Study an aecium and observe: — 

19. That it is sunken in the tissues; at first covered by the 
epidermis of the host (see one that has not burst through). The pycnia 
are more superficial; subepidermal or subcuticular? 

20. The fine interwoven mycelial threads between the host- 
cells about the aecia. Do they ever penetrate into the host -cells ? 

21. That it is from a densely inter\\^oven mat of this mycelium 
(to be seen at the base) that the structures of the aecium are developed. 

22. The closely packed club-shaped sporophores arising from 
the mycelial mat in the base of the cup. 

23. The chains of spores cut off from each sporophore, forming 
the parenchyma-like mass that fills the cup. Wh}' are the aeciospores 
here angular in outline? Note that the maturing spores at the mouth of 
the aecium become globose. 

24. The large cells lining the cup, — peridial cells. They also 
arise in chains from short mycelial branches similar to sporophores. Note 
the difference in thickness of the wall of the inner and outer faces of the 
cells. This lining of the cup is pseudoparenchymatous and is called the 
peridium. 

The mycelium of the aecial stage arises from the uninucleate sporidium 
and each mycelial cell is uninucleate. When the formation of the aecium 
begins, there is a sexual fusion of many pairs of mycelial tips in the region 
where the base of the cup is to be. From each pair of branches arises 
a single binucleate stalk, — a sporophore of the aecium. A chain of spores 
is developed from each of these binucleate sporophores by successive 
conjugate divisions of the paired nuclei and the laying-down of a septum 
between the resulting pairs thus formed. Examine stained sections 
carefully under high -power and locate : — 

25. The paired nuclei in the sporophores ; in the aeciospores. 
Make a drawing to show the structure of the aecium, pycnium and 

their relation to the host-tissue as seen in section. 

RemiOve with the needle or scalpel some mature aeciospores from the 
cups on diseased leaves provided. If dry, mount in potassium hydroxide. 
Study and observe: — 

26. Their form, size, color and contents; markings on the 
epispore. draw several aeciospores. 



151 

But one crop of aeciospores is produced. With the maturity and 
discharge of the last aeciospores, the activities of the primary cycles cease. 
The aeciospores initiate secondar^^ cycles on the cereal-hosts through which 
the pathogene perpetuates itself until the following year. There is no 
saprogenesis in the primary cycles. 

Secondary Cycles, especially the late ones, develop chiefly on the 
leaf-sheaths and stems of the wheat, oats or grass-hosts. Leaves may 
also become infected. 

Pathogenesis. The first secondary cycles are initiated by aecio- 
spores ^^■hich, falling upon the host, germinate in the dew or raindrops 
sending out a germtube which rapidly grows along over the siirface until 
its tip reaches a stoma through which it penetrates to the tissues within. 
The proto])lasm of the spore passes out into the germtube keeping con- 
stantly in its tip as it grows. Within the substomatal cavity, the tip of the 
germtube applies itself to one of the parenchymal cells and sends into it a 
short blunt branch, — a haustorium. 

Having thus established a food-relation with the host, a binucleate 
ramifying mycelium is developed. This mycelium produces mature 
uredinia within two or three weeks. 

Study cross-sections (freehand or prepared) of a stem through a uredi- 
nitmi and make out: — 

27. The structure of the sorus and its relation to the host -tissue. 

28. The intercelliilar mycelium. Can the globose haustoria be 
found ? 

29. The stout sporophores arising from the mycelium, each 
bearing at its apex a uredospore in som^e stage of development, or mature. 
How can one tell whether the spore is young or mature ? 

Make a drawing of a cross-section through the uredinium. 
Scrape some uredospores from a diseased stalk; mount in potassium 
hydroxide and observe : — 

30. The fomi, size, color and contents of the uredospore as 
compared with the aeciospores; markings on the epispore, if any. 

These uredospores are usually wind-disseminated, initiating other 
secondary cycles. They germinate at once in moisture, even in a moist 
atmosphere. They infect the host in a manner and under conditions 
similar to the aeciospores. The germtubes emerge through special pores 
or thin places in the spore-wall. Mount and examine tiredospores that 
have been treated with acetic acid, observe: — - 

31. The size, number and location of the pores, draw. 

For gemnnation and penetration into the host, stud}' illustrations 
provided. Supplement this with a study of germinating spores on slides, 
if available. Make drawings to show the points brought out. 

From, the mycelium produced from the uredospore, other uredinia 
with mature uredospores are produced. This may be repeated seven or 
eight timxcs during the growing-season. The mycelium remains binu- 
cleate throughout all the uredospore-generations, the spores being binu- 
cleate also. 

As the host begins to. mature, there is developed from the mycelium 
in the secondary lesions, not uredospores but another spore-form, — 
chlamydospores, black resting-spores known as teliospores (teleutospores). 
They often arise at first in the same sorus with the uredospores, later they 



152 

appear in sori in which no uredospores are formed. Mount some teliospores 
in potassium hydroxide and study : — 

32. Their form, size and structure as compared with uredospores; 
color, where located, in walls or contents? 

33. The thick walls and densely granular oily contents. 

34. The light spot in the center of each cell; not always evident. 
The nuclei are located here. These spores are at first binucleate but at, or 
just before germination the nuclei fuse and sexual fertilization, begun by 
the association of nuclei in the aecium, is now completed. 

The structure of the telium and its relation to the host-tissue does not 
differ materially from that of the uredinmm. draw several teliospores 
to show structure and variations in form and size observed. 

Saprogenesis. With the ripening or death of the host, teliospores 
have been formed usually in great abundance on the straw. When the 
grain is harvested, many teliospores are left on the stubble. Those on 
the straw go with it into the stack, then into the manure pile and finally 
with the manure back on the land. Under the conditions of moisture 
and temperature that starts the barberry into growth in the spring, the 
teliospores on the old straw become active. They germinate in situ form- 
ing a promycelium from each cell of the spore. Four basidiospores are 
produced from each basidium. Each is uninucleate, its nucleus having 
been derived through the reduction division of the fusion nucleus of the 
teliospore. These sporidia, as we have seen, constitute the primary inocu- 
lum which affects the young leaves of the barberry. They cannot, so far 
as is known, affect any of the cereals or grasses. 

REPORT 

1. Prepare a diagram showing the spore-forms and activities 
of Puccinia graminis Pers. in their proper sequence through the primary 
and secondary cycles. 

2. Explain the effect of the black stem-rust on the tissues of its 
host, and point out how it causes reduction in yield. 



This is a disease of European origin, which was first observed in this 
country in 1890. It rapidly developed in epiphytotic form, shortly appear- 
ing in every carnation house throughout eastern United States. It was 
for a decade the most destructive and most feared of carnation diseases, 
but is now enphytotic and rarely' serious, due to the development of 
resistant varieties. It also affects som.e other species of the genus Dianthus 
and species in related genera. In Europe the cluster-cup stage of this 
disease is known, affecting Euphorbia gerardiana Jacq. 

SYMPTOMS 

Since the cluster-cup stage of the carnation rust is unknown in America, 
the disease will be studied only as it appears on the common greenhouse 
carnation. The red and black rust stages appear together and are so 
much alike that the symptoms they induce will be considered together. 
Both leaves and stems are affected. Study the materials provided and 
OBSERVE : — 

1. The pale-green or yellowish areas on the leaves; which 
surface "" 

2. In some of these spots, dark-brown or black pustules, — 
sori of the pathogene. The lighter-colored ones are uredinia, the darker 
ones, telia. 

3. The aiTangement of these sori within the lesion; scattered, 
confluent, linear? 

4. Some of the sori burst open exposing the dark-colored rusty 
spore-mass within; the rtiptured epidermis forming a papery fringe. 

5. The sori on the stems; usually darker than those on the leaf. 
Stem-sori are largely telia. 

(). The general effect of the disease on badly rusted plants. 
Make drawings to show the characters of the lesions on leaves and 
stems. 

ETIOLOGY 

The causal organism is a urediniaceous ftmgus, Uromyces Caryophy- 
lUnus (Schrank) Winter. It is heteroecious, having all the spore-forms 
characteristic of the rust-fungi. 

Life-history, Being in this country a parasite of a greenhouse-crop, 
the fungiis is able to propagate itself continuously through secondary 
cycles. The primary cycle therefore will not be here considered. 

The Secondary Cycles are all initiated by uredospores from sori on 
leaves or stems of living plants (except of course when aeciospores occur). 
They are constanth' being initiated as uredospores are matured and 
disseminated. 

Scrape some of the uredospores from a sorus; mount and study. 
OBSERVE : — 

7. The form, size, color, thickness of wall and markings on the 
epispore. Treat some with acetic acid and see if the number and arrange- 
ment of the germpores can be determined, draw several uredospores. 

These uredospores are scattered by wind or splashed, in watering, to 
nearby healthy leaves and plants. They germinate very readily in water. 
Examine the germinating spores on the slides provided, observe : — 

153 



154 

8. The number of germtubes produced from each spore. Are 
they septate? 

9. The densely granular protoplasm crowded into the tips 
of the long germtubes. The empty germinated spores. Compare with 
those not germinated. 

DRAW two or mi ore gemiinated tiredospores. 

The germtube enters through a stoma and forces its tip between the 
palisade-cells, sending haustoria into them, thus establishing a food-rela- 
tion with the host. Make thin cross-sections of a leaf through a sorus; 
clear in chloral hydrate; study and observe: — 

10. The intercellular mycelium; often in gnarled and inter- 
woven meshes, especially in the intercellular spaces, prying apart and 
isolating the host-cells; densely granular, septate. 

11. The peculiar lobed haustoria in the host-cells; their very 
thin walls and densely granular protoplasm. 

12. The urediniuni; its structure and relation to the host- 
tissue. 

13. The matted mycelial bed from which arise the short closely 
crowded uredosporophores. 

14. The globose uredospores; thin walled, covered with short 
spines and naich lighter colored than the one-celled teliospores, some of 
which are u.sually to be found in a uredinium. 

Make a drawing of a portion of a cross-section to show the pathogene- 
structures and their relation to the host -tissue. 

The m.ycelium is supposed to spread, very generally throughout the 
leaf- and stem-tissues beyond the point where distinct lesions appear, 
so that cuttings from diseased plants are usually invaded. It is believed 
that this habit of the pathogene is responsible for its wide and rapid distri- 
bution in this countr\'. Make thin sections through the leaf at some dis- 
tance from the lesion ; clear in chloral hydrate ; examine and determine : — ■ 

15. Whether or not mycelial invasion is general. 

Select a dark-colored sorus, — a teliimi (usually more abundant on 
stems); mount some of the tehospores. Study and observe: — 

16. That they are n:\uch darker in color than the uredospores, 
although of about the same size and shape; walls slightly thicker, and 
epispore nearly or quite smooth; stalk often still attached, draw several. 

The teliospores arise from the same mycelium on which uredospores 
are produced, usually miore abundantly in the sori formed as the affected 
organ grows older. 

Saprogenesis. The teliospores do not germinate at once as do 
the uredospores, but require a period of rest. They are chlamydospores, 
and, under natural conditions, germinate in the spring producing a promy- 
celium with four sterigmata, each bearing a basidiospore (sporidium). 
Study the drawings provided, copy. 

These sporidia produced by the germination of the teliospores can, 
so far as kno^vn, infect only the Euphorbia host. As Euphorbia gerardiana 
does not grow in this country, the sporidia cannot function in the propaga- 
tion of the pathogene, and primary cycles cannot develop. 

REPORT 

1. Prepare a diagram showing the complete life-history of 
Uromyces Caryophyllinus (Schr.) Wint. as it may occur in Europe. 



DODDER 

The term dodder is applied as a common name to the pathogenes as 
well as to the diseases which they cause. It is used for all the diseases 
caused by sp.ecies of phanerogamous parasites of the genus Cuscuta. 
The different dodder diseases are often distinguished by adding the host- 
name as, alfalfa dodder, clover dodder and the like. The pathologic 
eft'ects are in all cases similar. 

SYMPTOMS 

All parts of the host above ground, especially the stems, are directly 
attacked by dodder jjathogenes. The most striking sign is the dodder 
parasite itself. The tangled yellow stems matted about its host form 
conspicuotis yellow patches in the field. These doddered areas, often 
several feet in diameter, are at first more or less circular. Examine the 
photographs showing spots in the field, observe: — 

1. The dwarfed and dying host-plants at the center of the dod- 
dered area. 

2. The densely matted dodder stems. 

3. The long spreading runner-like dodder stems about the 
margins of the area, extending otit among the healthy plants. 

Examine the specimens provided and observe: — ■. 

4. The yellow dodder vine entwined about the host-stems; 
size, color, shape in cross-section, branching. 

5. Evidences of injury to the host, to be detected by comparison 
of diseased and healthy plants; dwarhng and chlorosis. 

Make a drawing to show the general appearance of a dodder-affected 
plant. 

ETIOLOGY 

Dodder diseases are caused by species of Cuscuta, a genus of the family 
Convolvulaceae. Some near relatives in the same family are the morning- 
glories, bindweeds, buckwheat and sweet potatoes. There are about 
ninety species of Cuscuta, of which about half occur in America. The 
most cominon one on cultivated crops like clover and alfalfa is Cuscuta 
epithymum Aiurray, a species introduced from Europe. 

Life-history. This pathogene has an annual life-history. It is known, 
however, that it commonly overwinters in a vegetative condition on 
perennial hosts like clover and alfalfa. (See N. Y. (Geneva) Bui. 305 : 369- 
374.) 

The Primary Cycles are initiated by the seeds which have lain on 
the ground over winter. . 

Pathogenesis. Examine seed of C. epithymum. note:— 

6. Their surface markings; color; and that the one side is 
convex while the other is flattened in two or more places. Measure 
several, draw: — (a) several to show variations in size and form; (b) two 
seeds greatly enlarged showing all their characteristics. Describe the 
color in the notes on the margin of the paper. 

7. Examine the seed of the other species provided. These 
may all occur on clover or alfalfa. Make enlarged drawings of each 
so that they will be comparable in size with those of the drawings of C. 
epithymum. 

155 



156 

S. The embryo is coiled in the seed and imbedded in the food 
substance. Examine the photographs of plates taken from Koch's work. 

DRAW. 

9. If available, examine the dodder seedlings and note how 
they first attack the host-plant. Examine N. Y. (Geneva) Bui. 305, 
pi. 3, and Koch's pi. 1, fig. 1-3. Make drawings to show the characters 
of the seedling and its method of attacking the host. 

As soon as the seedling has attached itself to the host, the vestigial 
root by which it was anchored in the soil, shrivels and dies. The twining 
stem, however, feeds on the host, grows rapidly and by branching 
spreads to neighboring hosts. Study the dodder plant on the host pro- 
vided. OBSERVE : 

10. The type of branching; opposite or alternate? 

11. The leaves; their location, size, shape, color and abundance. 
How are these foliage conditions to be accounted for? 

Untwine and separate a portion of the dodder thread from the host- 
stem. OBSERVE : — 

12. That it is attached at some points. At such places the spirals 
are shorter and are tight about the stem. 

13. With the hand-lens, the rows of minute punctures in the 
host -stem where the dodder was attached; the oval depression about each. 

14. With the hand-lens, the corresponding row of concave 
sucker-like disks on the dodder stem, — the attachment -disk ; the minute 
elevation at the center of each disk, — the broken-off haustoriiun. 

15. The aborted haustoria in places along the dodder stem 
where it failed to come in contact with the host. 

Make a drawing of a portion of a host-stem with entwined dodder 
to show all the points brought out in 10-15. 

Cut thin cross-sections through the clover and dodder stems at the point 
where haustoria are attached. Cover and examine under the low-power. 
OBSERVE : — 

16. In the clover stem, the large pith, the vascular cylinder, 
the cortex, and the epidermal layers. In the vascular cylinder, note the 
xylem containing the large ducts, and the phloem which consists of soft 
sclerenchyma within and the lighter area of sclerenchyma-fibers without. 

17. In the dodder stem, the haustoriimi. Note the shape of 
the attachment-disk in cross-section; its relation to the haustorium; 
where the haustorium arises and how it penetrates the host. Note what 
happens where it comes in contact with the sclerenchyma-fibers. Toward 
which tissue of the host-stem does it grow? Why? Note the spiral 
ducts in the center of the haustorium. 

Make diagrammatic drawings of the host-stem and the dodder stem 
to show the relative sizes and relation of parts. 

With the low-power, examine the stained longitudinal sections of the 
haustorium of dodder on Impatiens (touch-me-not), note: — 

IS. The long thread-like cells extending out from the tip and 
sides of the haustorium into the cortex, pith, and sometimes into a bundle, 
making a connection with a conductive vessel of the host. 

19. The layer of crushed cells at the point where the haustorium 
emerges from the dodder stem. Does the epidermis of the dodder con- 
tinue about the haustoriimi? What becomes of it? Are the cells of the 
host crushed? What is the function of the thread-like cells? 



157 

DRAW a single haustorium showing the type of cells in the center and 
at the ends; the connection of the host and the dodder. Toward autumn 
flowers may be developed on the dodder vine. 

Study a fruiting branch of the dodder and observe: — ■ 

20. The compact flower-clusters. Where do they seem to be 
most commonly formed? How many flowers in a cluster? 

21. Under the hand-lens, the scale-like leaf at the base of each 
flower-cluster and at the base of each flower. 

DRAW a single flower-cluster enlarged three times. 
Dissect and study a single mature flower, note : — 

22. The structure and an-angement of the cal}Tc, corolla, sta- 
mens with fringed scale at the base of each, and the pistil. How many 
parts of each? draw a single flower enlarged, with calyx and corolla 
removed at one side. 

Cut across the ovary, examine with hand-lens and determine: — 

23. How many carpels and ovules are produced, draw. 
Examine the matured capsule. How does it compare in size with the 

flower? How m.any seeds does it contain? Note the remnants of 
the calyx or the corolla at the base. From what does the capsule develop? 

DRAW. 

Saprogenesis. The seeds fall to the ground where they lie dor- 
mant until the following spring. They germinate when conditions favor- 
able to the growth of the host prevail. The vestigial root serves as an 
anchorage for the simple slender seedling, probably taking up only water 
from the soil. The seedling seems to he dependent largely on food 
stored in the seed, although chlorophyl is slightly developed. Seed 
germination and contact of the seedling with its host completes the pri- 
mary life-cycle. 

Secondary Cycles may be initiated during the growing-season by 
pieces of dodder stems broken off in cutting and harvesting the host, and 
falling upon healthy plants, infecting them. Pieces of dodder stems are 
very tenacious of life and thus afford a dangerous inoculum for spreading 
the pathogene in cultivated crops. The spreading of the pathogene from 
one individual host -plant to a neighboring one may also be regarded as a 
type of secondary infection. 

REPORT 

1. A farmer had a few spot-infections in an alfalfa field. After 
cutting the hay, the ground where the dodder occurred was spaded. Soon, 
however, the dodder appeared about the margins of the same areas. 
Write a letter outlining the measures to be taken to prevent further 
local spread of the pathogene and its dissemination over the field. 

2. Give directions for determining whether or not there is 
dodder in the alfalfa seed, and if present, how to get rid of it. 



TOBACCO MOSAIC 

This is a disease not only of tobacco but also of many other solanaceous 
plants. 

Plants affected with the mosaic disease, or as it is often called, calico, 
mottled-top, grey-top, or frenching, do not usually die. There is pro- 
nounced hypoplasia, especially of young leaves. There is a reduction 
in size, and chlorophyl-development is halted, so that a true chlorosis 
results. 

SYMPTOMS 

The leaves, and sometimes the branches, are the only organs of the host 
which exhibit symptoms. The plants are often much dwarfed and, in the 
case of Nicotiana rustica L., are sometimes killed. In some of the so- 
called immune varieties, it has been found that a progressive rotting of the 
tissues follows inoculations with the virus, but in such cases there are none 
of the usual symptoms of the disease. (U. S. Agr. Dept. Bui. 40: 2.) 

The symptoms vary from small blisters on the leaves, or the distinct 
dark-green islands in large areas of yellow, to an excessive suppression 
of the development of the lamina of the leaves, in which only the midrib 
remains normal. 

On the entire plant. Study the diseased and healthy tobacco plants 
provided . observe: — 

1. The comparative size and appearance of the healthy and 
diseased plants, sketch or copy from U. S. Agr. Dept. Bui. 40, pi. VII. 
Maintain relative proportions between the healthy and diseased plants. 

On the leaves. Examine the plants provided and locate, but do not 
remove, leaves showing various symptoms, observe : — 

2. The characters of a healthy leaf, its regularity of surface 
and margin; its size. draw. 

3. The peculiar green and yellow blotched appearance of diseased 
leaves. Note the blister-like elevations on the upper surface. Are they 
swellings or are they archings of the leaf? In which areas do they occur? 
Is there any relation between the veins and the green areas ? 

4. That the yellow areas of the leaf are apparently thinner 
than the green islands. 

5. The comparative size of healthy and diseased leaves of the 
same age. 

DRAW a diseased leaf on the same scale as the healthy leaf. Bring out 
in the drawing and labeling as m.any of the characters as possible. 

6. The distorted leaves. These may be long and ribbon-like 
or the lamina of the leaf may be entirely undeveloped, although the mid- 
rib is not affected, draw, or copy U. S. Agr. Dept. Bui. 40, pi. II, fig. 1. 

7. That certain upper leaves may show a mottling (green and 
yellow) but without the accompanying distortions of the surface. Note 
how the green areas follow the veins. This is the phase that gives the 
names, grey-top and mottled-top, and occurs when the plant becomes 
infected just as the immature flower-heads appear, draw, or copy U. 
S. Agr. Dept. Bui. 40, pi. I, fig. 2. 

On the blossoms. Examine the flowers of the plant provided, or 
XJ. S. Agr. Dept. Bui. 40, pi. Ill and IV. Various species of Nicotiana 

158 



159 

are shown. Note particularly the pink-flowered form of A^. Tabacum L. 
and A^. paniculata, if available, observe : — 

S. The mottled or blotched appearance of the corollas of A^. 
Tabacum. Note the variations and compare with the flowers of healthy 
plants. 

9. That some of the flowers show a depauperate growth of 
the corolla; the stamens and stigma extrude. 

10. That the buds may be depauperate and variously mis- 
shapen. 

1 1 . That the corolla-tubes of A^ paniculata are bent and crump- 
led. 

Make drawings of flowers and buds, both healthy and diseased, to 
show the various symptoms. Indicate all points, which cannot be shown 
in drawings, by notes in the margin. 

On tomato leaves. Examine the tomato plant and observe : — 

12. The mottling of the leaves; their texture and flexibility; 
their size and shape, draw and note characters in the labeling. 

On petunia leaves, observe: — 

13. Their color, size and flexibihty. draw and label fully. 
The symptoms on other hosts are similar to these. 

ETIOLOGY 

The cause of this disease is unknown but there are various theories 
formulated to explain it. Some investigators (Woods, Chapman and others) 
believe that enzymes play an important, if not the primary, part in causing 
it; oxidase and peroxidase are specifically named. It is said that rough 
handling of the plants, such- as breaking the roots in transplanting or 
severe pruning of the tops, will cause the disease. Disturbance of the 
equilibrium between root-absorption and leaf -transpiration is also held to 
be responsible for mosaic. 

A mmch more j^lausible theory is that recently offered (by Allard and 
others). The disease is attributed to a filterable virus. It is held that 
this virus consists of an ultra-microscopic organism, bacterial or proto- 
zoan in nature. The extreme minuteness or plasticity of the organism 
probably allows it to pass through the pores of the Chamberland filter. 
Such phenomena are known in other cases. 

Crush small pieces of healthy and diseased tobacco leaves in small 
drops of water, cover, and examine each with the high-power, observe : — ■ 

14. The absence of any evidence of an organism in the diseased 
tissues. 

Pathological Histology. The effects on the structure of the leaf are 
very pronounced. INIake thin cross-sections through diseased and healthy 
tobacco leaves, or if available study prepared slides, observe : — 

15. The arrangement and contents of the cells of the palisade- 
layers, spongy parenchyma and epidermal tissues of the healthy leaves. 
draw in detail a section across the leaf. 

16. The corresponding cells in the green and yellow area of the 
diseased leaf. Note the suppression of the palisade-tissue in the yellow 
areas. The relative thickness of the green and the yellow areas, draw 
a section across a diseased leaf at the same enlargement as for the section 
across the healthy leaf. 



160 

Pathogenicity Studies. Although the cause of the disease is not known, 
it is ver\- easy to inoculate and obtain infections with the virus. A series 
of infection-experiments will be undertaken by the class as follows: 

General instructions. Students will work in pairs, A, B and C, as 
assigned by the instructor. There will be four experiments, each involv- 
ing the cooperative action of three pairs of students. Two pairs in each 
experiment will inoculate the healthy plants provided as directed, while 
the third pair will prepare check-plants. 

Healthy tobacco, tomato and petunia plants will be provided. Do 
not touch these plants except when directed and then exactly as directed. 
Experiment I. Inoculation by touching, proceed as follows: — 

17. Thoroughly wash the hands and disinfect in alcohol pro- 
vided. 

18. Pairs A and B, squeeze and rub but do not crush, between 
th-umb and finger, a diseased tobacco leaf; then immediately squeeze 
and rub with the same thumb and finger, leaves on the healthy plants. 

LABEL. 

19. Pair C, do not touch diseased plants but squeeze and rub 
with thumb and finger (do not crush), the leaves on the healthy plants 
provided for checks, label. 

Experiment II. Inoculation with sap from diseased plants, pro- 
ceed as follows: — 

20. Pairs A and B. Each pair will thoroughly clean and rinse 
a mortar in sterilized distilled water. Grind three diseased tobacco leaves 
in a little sterilized distilled water in the mortar. The virus is thiis freed 
from the leaf-tissue. Then very thoroughly wash the hands and disinfect 
in alcohol provided. Flame and cool a needle. Dip the point into the 
virus and prick the midrib of several yotmg leaves of the healthy plants 
provided. Dip the needle each timx before pricking, label. 

21. Pair C, thoroughly wash the hands and disinfect with 
alcohol. Do not touch diseased plants. Flame and cool a needle. Dip 
it in sterilized distilled water and prick the midrib of several yoimg leaves 
of the healthy plants provided, label. 

Experiment III. Root inoculation, proceed as follows: — 

22. Pairs A and B, thoroughly wash the hands and disinfect with 
alcohol provided. Do not touch the tops of the plants. Sterilize a 
scalpel by dipping in alcohol and flaming. Thrust the scalpel into the soil 
about the healthy plants provided, so as to cut some of the roots. Pour 
into the soil, where the roots were cut, some of the diseased-leaf extract 
prepared by pairs A and B in experiment II. label. 

23. Pair C, thoroughly wash the hands and disinfect with alcohol. 
Proceed as in number 22 but use sterilized distilled water to pour on the 
roots, instead of the diseased-leaf extract, label. 

Experiment IV. Inoculation through wounds, proceed as follows: — 

24. Pairs A and B, thoroughly wash the hands and disinfect in 
alcohol provided. Break a yoimg leaf from each of the healthy plants 
provided. Rub a diseased leaf between thumb and finger and touch the 
broken leaf-stub on the healthy plants but do not touch them elsewhere. 

LABEL. 

25. Pair C, thoroughly wash the hands and disinfect with 
alcohol. Do not touch diseased plants. Break a young leaf from each 



161 

healthy plant provided. Rub the healthy leaf between thumb and fingers 
and touch the broken leaf-stub with the finger, label. 

These plants will be kept in the greenhouse and are to be examined every 
two days for two weeks. Make notes on the first appearance of the dis- 
ease, i. e. (a) the number of days since inoculation; (b) the maturity of 
the infected leaves; (c) whether or not inoculated plants show infection. 

After two weeks, the plants will be returned to the laboratory' for final 
notes. At this time a tabulation-sheet of the restdts recorded will be made 
to obtain the class-result on the different types of inoculation. 

REPORT 

1. Give the class-record of the inoculation-experiments. 

2. State the various ways that the virus can be disseminated. 
What precautions should be taken to prevent dissemination. Explain. 



[ETAPLASTIC DISEASEl 



SPONGOSPORA SCAB OF POTATOES 

This slime-mold disease has come into considerable prominence in 
recent years because of its discovery in the seed-growing potato-sections 
of northern United States and Canada. It appears to be confined to the 
potato. Soil and climatic conditions seem to greatly limit the geographical 
range of the disease. It is evidently a disease of cool, temperate climates 
or high table-lands and valleys. 

SYMPTOMS 
The lesions of this disease are confined to the underground parts of the 
host; tubers, roots and stolons (rhizomes). 

On the tubers. Three types of lesions are to be found on the tubers; 
the scab (the most common form), the wart -form and the canker-form. 

Scab-form. Examine the specimens labeled "young sori." ob- 
serve : — 

1. The small brownish spots (ringed with India ink). These 
are the first evidences of the disease. 

2. The smooth, brown mounds (also ringed with ink), — the 
covered sori of the pathogene. 

3. The crater-like openings of some of the sori; they are mature 
and are opening for spore-dispersal; form, size and color of the sori. 

Examine the specimens labeled "mature sori." observe: — 

4. The large open brown sori. Note the torn and upraised 
cuticle about the margin. 

5. The brown powder within the sorus. Pick it out with the 
needle. 

6. The erripty depressions left after this powder is disseminated. 
Make a drawing of the entire tuber to show the sori and their distri- 
bution. 

Make a series of enlarged drawings (at least four), showing the ap- 
pearance of the sori in different stages of development. 

Wart-form. This form is not common but may develop in severe 
cases in moist soils. Read Maine Bui. 227:94, and study figure 45; or 
study specimens if available, observe: — 

7. That the lesions are more or less confined to the seed- or eye- 
end of the tuber. 

8. That they are much more enlarged and gall-like than the 
ordinar}' scab-form. How does the location of the lesions help to explain 
their warty character? 

SKETCH a tuber showing the wart-form of the disease. 

Canker-form. Where the lesions are numerous and coalesce, the 
tubers may show, in an advanced stage of the disease, large deep 
cankers. This is held by some to be the result of secondary activities of 
the pathogene. (See Kunkel, Jour. Agr. Research 4:237.) Study the 

162 



163 

illustration specimens provided; Maine Bill. 227, fig. 44; and U. S. Agr. 
Dept. Bui. 82, pi. III. observe: — 

9. The large open cankers, extending deeply into the tuber and 
lined with the dead and decaying tissues of the host, draw a tuber 
showing this form of lesion. 

On the roots and stolons. Study the specimens provided, observe : — 

10. That here the lesions are gall-like in appearance; form and 
size; relation to the root or stolon, i.e. on the side only or involving the 
entire diameter !* draw. 

Comparative studies. There are a number of scab-like diseases of 
potato tubers, the lesions of which are more or less alike. A brief examina- 
tion of the more common tuber diseases, in comparison with the Spongo- 
spora scab-lesions, is important. Examine, so far as available, the speci- 
mens and illustrations of the following : — 

11. The black-wart disease. In addition to specimens and 
illustrations provided, study PL Ind. Bur. Circ. 52, pi. I-II. sketch 
a tuber to show the character of the lesions of this disease. 

12. The common scab. Note the slightly raised irregular corky 
scab-spot. If this disease is serious, the tuber may be much checked and 
cracked, draw. 

13. Silver-scurf. This disease does not cause a distinct scab; 
at most there is but a wrinkling or sloughing-off of the epidermis. (See 
Schultz, Jour. Agr. Research 6:345-346.) 

14. Black scurf or Rhizoctonia scab and canker. This disease 
is very common but not often directly injurious to the tubers. Note 
the small dark-brown or black sclerotial bodies of the fungus scattered 
over the tuber. It is held by some that checking of the surface and even 
the formation of deep cankers are sometimes symptoms of this disease. 
(See specimens or Maine Bui. 230, fig. 71-73.) Make drawings to show 
the tuber-symptoms of this disease. 

ETIOLOGY 

The Spongospora scab is caused by the myxomycete, Spongospora 
subterranea (Wallroth) Johnson. It is generally distributed throughout 
northern Europe, Canada and northeastern United States. It is a native 
of South America where it occurs as a pathogene of the potato in its native 
home. 

Life-history. This slime-mold does not differ materially from the 
saprophytic myxomycetes in its structure and life-history, except in so far 
as it has been modified by its parasitic habit. 

Primary Cycle. Spore-balls in the soil constitute the source of 
inoculmn for the primary infections. The spores may apparently remain 
donnant in the soil for several years. Remove some of the spore-mass from 
a sorus on the tuber provided; mount in potassium hydroxide and 
observe :— 

15. The rather large dark spore-balls; color and structure. 
Each cell of the spore-ball is a spore, draw. 

Pathogenesis. The spores in a single spore-ball all germinate at 
about the same time. There emerges from each cell of the spore-ball, 
a single, usually uninucleate ameba. (See Kunkel, Jour. Agr. Research 4,- 
pi. XXXIX, fig. 1-2. COPY). 



164 

These amebae soon fuse to form a small Plasmodium which penetrates 
through and between the epidermal cells of the tuber. If conditions are 
not favorable to the formation of a plasmodium, the individual amebae 
may encyst. Later, when conditions become favorable, each protoplast 
escapes from its cyst-wall through a hole and may fuse with others to 
form the plasmodium. 

Once within the tissues, the plasmodium spreads out under the epidermal 
tissues and down between the cells of the cortex. Study the prepared 
slides provided; or Jour. Agr. Research 4, pi. XXXIX, fig. 3-5, and pi. 

XL. OBSERVE : — 

16. The densely granular protoplasm of the plasmoditim in the 
intercellular spaces just beneath the epidermal layers. 

17. The pseudopodia-like extensions of the plasmodium, forcing 
their way down between the cortical cells. The middle-lamella is dis- 
solved and the cells are forced apart. The walls become soft and swollen. 

18. That the pseudopodia penetrate the cell-walls and portions 
of the Plasmodium enter; these become separated from the main body of 
the Plasmodium. 

Make a drawing to show the invading plasmodium, outlining the 
adjacent host-cells. 

After a certain period of feeding, all the plasmodia in the host-cells 
of a particvilar lesion simultaneously^ form spore-balls. Examine prepared 
sections through sori showing mature spore-balls; or study U. S. Agr. 
Dept. Bui. 82, pi. I. observe: — 

19. That the entire plasmodium is converted into spores. 
The host-cell serves as a sporangium. 

20. The average number of spore-balls in a host-cell. 

21. Remnants of the host-protoplast; nucleus, starch-grains, 
or cytoplasm. 

22. Host-cells in which spore-formation is incomplete. Com- 
pare nuclei and cytoplasm of the pathogene and host. 

Make enlarged and detailed drawings of the host-cells, (a) containing 
Plasmodia; and (b) containing mature spore-balls. 

Saprogenesis. What takes place after the spore-balls are set 
free in the soil is not certainly known. They probably germinate very 
shortly, the amebae passing out into the soil where they either form a 
Plasmodium or encyst separately. Whether the organism can live a 
true saprophytic life in the soil is not known. It has, however, been 
cultured on artificial media. 

Secondary Cycles probably occur, as the spores can apparently 
germinate as soon as they are mature. The phenomena of the secondary 
cycles are probably the same as those of the primary cycles. A peculiar 
type of secondary infection of the deeper tissues of the tuber, surrounding 
a mature sorus, has recently been described as the cause of the canker- 
form of the disease. (See Kunkel, Jour. Agr. Research 4:272-273.) 

Pathological Histology. There is a marked stimulation of the host- 
tissues by Spongospora subterranea, as may be seen by the comparative 
study of the cells in the diseased and healthy regions of the tuber. 

Cut thin cross-sections (or use prepared slides) through the healthy 
potato skin. Examine and observe: — 

23. The single layer of thin cells on the outside, — the epidermis, 
overlaid by the brown cuticle. 



165 

24. Beneath the epidermis the layers of the cells which are 
regularly rectangular in shape. This is the corky hypodermis. The 
epidennis and hypodermis constitute the skin of the potato. 

25. That from the hypodermis there is a rather abrupt transition 
to the larger, more globose cells of the cortex. Note that the cells of the 
cortex are filled with starch. 

Make a drawing of the skin and cortex of the healthy potato. 

Cut thin cross-sections (or use prepared slides) through sori of different 
ages; young and medium old. Put the sections from each into different 
watch-glasses. 

Mount the thinnest sections from the very young sori and note : — 

26. Whether the hypodermis and epidermis are affected; the 
cells where the Plasmodium enters are killed. 

27. The granular mass between and within the cells, — the 
Plasmodium of the parasite. 

28. Any discoloration of the cells. Is there any enlargement 
of the cells? Note the starch-granules. Iodine solution will stain the 
starch-grains and make them more evident. 

Examine the sections through the medium-aged sori. observe: — 

29. The hypertrophy of certain of the cells; enlarged in which 
direction? These are called giant-cells. Is the plasmoditmi of the patho- 
gene to be detected in any of them? Has spore-formation begun? 

30. That some of these giant-cells are divided by cross- walls 
into several smaller cells; hypoplasia occurs as well as hypertrophy. 

31. That this overgrowth of the cortical cells pushes up and 
eventually bursts through the skin. 

32. Any effect of the pathogene upon the starch. 

33. Effect on the nuclei of the host-cell. 

34. Evidences of cambial activity in the uninvaded tissues 
beneath the sorus. 

Make a drawing of a section through a sorus showing the histological 
conditions just prior to spore-formation. 

REPORT 

1. Give the methods of control to be employed against this 
disease and explain the philosophy of their use. 

2. Consult the literature on club-root of crucifers and write a 
brief description of the life-history of Plasmodia phor a Brassicae Wor. 
comparing and contrasting it with that of Spongospora subterranea 
(Wallr.) Johnson. 



CLUB=MOOT OF CEUCIFERS 

This is the best known of the sHme-mold diseases of plants. It is a 
distinctly metaplastic disease, causing a marked overgro'wi;h of the affected 
organs. It is widely prevalent both in the Old and in the New World. 
It affects most of the common cruciferous crops, especially cabbage, 
cauliflower and turnips. 

SYMPTOMS 

The roots are the only organs of the host that show lesions. The 
specimens provided exhibit the symptoms at different stages in the develop- 
ment of the disease and on different hosts. (See illustration speciinens 
of the disease on various hosts.) Examine carefully the specimens pro- 
vided and observe: — 

1. The relatively small size and the much more fibrous and 
branched character of the roots of the healthy plants as compared with the 
diseased roots. 

2. The much swollen and enlarged roots of the diseased plants; 
occasional healthy roots arising from the stems above the lesion. Why? 
Determine by diameter measurements how many times the diseased root? 
have been enlarged. Roots enlarged in this manner are said to be hyper- 
trophied. 

3. That the hypertrophy occurs either in the region of the root 
next to the stem or out in the feeding-roots. 

4. That the hypertrophy involves the root in its entire diameter. 
It is not simply an outgrowth on the side of the root. 

5. The specimens of club-root on the different hosts in the illus- 
tration jars. (See also Vermont Bui. 185, pi. II-V.) Determine whether 
there is any uniformity in the shape of the swellings which may be regarded 
as characteristic of this disease. 

6. That the surface of the hypertrophied roots, especially in 
the old specimens, is much cracked and checked; to what due? These 
cracks allow the entrance of decay -producing fungi and bacteria. The 
succulent tissues quickly become soft and rotten and have a foul odor. 
The club-root pathogene itself does not cause the tissues to soften. 

7. That the root beyond the hypertrophy, the branches and fine 
feeders, seem to be normal and healthy. 

8. The abruptness with which the hypertrophy ceases at the 
base of the stem ; this is a disease of the roots. 

Nodules, tubercles and galls due to other causes are often produced on 
roots of plants, as for example: the nematode-gall caused by minute 
worms; legume-tubercles and crown-gall of trees caused by pathogenic 
bacteria. (See illustration specimens in jars.) 

Make a drawing of a "clubbed" root along side of a healthy one. Main- 
tain relative proportions. 

Where the roots are badly clubbed, the plants fail to head, or form loose 
worthless heads. Affected plants often wilt or "flag" during the heat of 
the day but recover at night. These symptoms exhibited by the tops 
can be well observed only in the field. 

166 



167 

ETIOLOGY 

This disease is caiised by the myxomycete, Plasmodiophora Brassicae 
Woronin. It is apparently an obhgate parasite, saprogenesis being merely 
a period of rest. It has never been cultured saprophytically. 

Life-history. This pathogene has, normally, an annual life-history 
but its spores may evidently lie dormant in the soil for se^'•eral years in 
the absence of a cruciferous crop. 

The Primary Cycle is initiated by spores that have remained dormant 
in the soil from a former season. 

Pathogenesis. When the affected roots rot, the spores of the patho- 
gene, mixed with the decayed host-tissue, remain in the soil. Selecting a 
root in an advanced stage of the disease, remove a bit and crush it in a 
drop of water on a slide; cover and examine with the high-power, ob- 
serve : — 

9. The minute globose hyaline spores, abundant among the 
crushed tissue-fragments, draw. 

These spores germinate in the spring after the manner of those of the 
saprophytic slime-molds. The spore-wall cracks and the naked protoplast 
creeps out. Study the process as illustrated in Vermont Bui. 175, fig. 6. 
COPY to show several stages in spore-germination and the development 
of the swarmspore or myxameba. 

These m^ocamebae, thus set free in the soil water, come in contact 
with nearby roots of susceptible plants and infection results. The single 
uninucleated swarmspore enters through a root-hair. Here by rapid 
nuclear division and growth, a multinucleate ameba or plasmoditim is 
formed which penetrates into the cortical tissues. By direct migration 
through the cell-walls and by the division of the meristematic host-cells 
into which the ameba have penetrated, invasion of the various tissues is 
effected. 

The Plasmodium increases in size, destroys the protoplast of the cells 
in which it is lodged, and gradually fills the lumena of the cells. Make thin 
sections (or use prepared slides) through roots in the early stages of the 
disease, observe : — 

10. The finely and densely granular multinucleate protoplasm 
of the pathogene in certain of the enlarged cortical or medullary ray-cells. 
Compare with the protoplasm of vininvaded cells nearby. (See Vermont 
Bui. 175, fig. 3-4.) DRAW an uninvaded and a normal host-cell, with 
contents, to show contrast. 

These multinucleate plasmodia, when mature, divide into spores, each 
one-celled. Cut sections (or use prepared slides) of diseased roots con- 
taining mature spores, observe: — 

11. That the infested cells are packed with spores; uniform size 
of spores; and absence of capillitium, characteristic of most saprophytic 
slime-molds. The host-cell serves as a sporangium, draw an invaded 
cell with spores. 

12. In which tissues of the root the pathogene is most abundant. 
Saprogenesis. The affected tissues gradually decay and the 

mature spores go into a resting- or dormant-period. No saprophytic 
activities, between spore-germination and infection by myxamebae in the 
spring, are known to take place. 

Secondary Cycles. It is known that the spores will germinate at 
once or very shortly after they are formed. Therefore, if set free by the 



168 

early decay of affected tissues, they may cause secondary infections on 
roots of the same or later-planted hosts throughout the season. That this 
commonly happens appears certain. The phenomena are the same as 
those of the primary cycle. 

Pathological Histology. Make freehand sections of a small healthy 
root (or use prepared slides). With low-power, observe: — 

13. The core of the root, made up of alternating wedges of 
xylem (vessels and wood-cells) and medullary rays. Note that the xylem- 
wedges are frequently fused toward the center of the stele so that the medul- 
lary rays are seldom continuous from the cortex to the center. 

Stain the section with a solution of iodine and note : — 

14. The yellow color taken on by the walls of the wood-cells 
which, with their thicker walls, are now more sharply contrasted with 
the medullary ray-cells. 

15. The bark of the root surrounding the xylem-cylinder con- 
sisting in large part of the thin-walled cortex-tissues. 

16. The groups of thick -walled sclerenchyma-cells in the outer 
cortex, stained deep yellow by the iodine. What is their function? 

17. The ends of the phloem-strands, made up of small thin- 
walled cells in the inner cortex, each opposite a xylem-wedge. 

IS. The outermost tissues of the root, rough and torn, — the 
cork-layer. 

Make a diagrammatic drawing three inches in diameter showing 
the anatomy of a healthy cabbage root as seen in cross-section. Make an 
enlarged and detailed drawing of a V-shaped portion of the cross-section 
showing form and arrangement of cells from center to circumference. 
Include (in outline) at least one vascular wedge. 

Cut, mount and study a thin cross-section of a diseased root (or use 
prepared slide), observe:— 

19. The abnormally increased diameter of the root. To what 
does this seem to be due; increase in size (hypertrophy) or numbers 
(hyperplasia) of the cells or both? (Compare with the section of the 
healthy root.) 

20. The xylem-vessels in a few small groups. Note their isola- 
tion. To what part of the section are they chiefly confined? Why? 
(Compare Vermont Biil. 175, fig. 3.) 

21. The ends of the phloem-strands composed of small regular 
cells arranged in rows around the outer end of the xylem-wedge. The 
relatively large amount of phloem, as compared with the amount of xylem; 
as compared with the amount of phloem in a bundle in the healthy tissue. 

22. The abnormally broad medullary rays, resulting from the 
increased size of the cells. The dark masses in some of the cells, — the 
Plasmodia or spore-masses of the parasite. 

23. The cortex; the most abnormally increased of all the tissues. 
The cells are enlarged and perhaps increased in number. Some of them 
are filled with the brown masses of the parasite. Most of them show 
nucleus and cytoplasm. Note the entire absence of sclerenchyma or 
stone-cells ; why wanting ? 

24. The layer of cork-cells on the outside. Are they affected? 
Make a diagrammatic drawing showing the anatomy of the diseased 

root as seen in cross-section. 



169 

Make an enlarged and detailed drawing of a V-shaped portion of the 
section from the center to the outside, including one vascular wedge and 
medullary ray. Indicate the presence of the parasite in the cells by fine 
stipple-work. 

REPORT 

1. Describe fully the effect of the disease on the morphology 
of the root. What is meant by pathological morphology ? 

2. Explain the difficulties involved in the control of this disease 
by eradication measures. 



LEGUME TUBERCLES 

The bacterial tubercles which commonly occur on the roots of legumes 
are not generally regarded as evidence of a diseased condition. They are 
nevertheless pathologic in character. That the plant usually profits 
from their presence on its roots is no argument against their pathologic 
nature. The roots of all the common leguminous crops usually show these 
galls. 

SYMPTOMS 

Roots of various legtiminous plants have been provided. (See also 
the illustration specimens in jars.) Study and compare the tubercles 
on the different hosts, observe: — 

1. The shape, size and color of the galls; more or less characteris- 
tic for different hosts. 

2. Location on the root-system; deep or shallow; on small 
or large roots; lateral or terminal? 

3. Types of galls; simple and compound (branched). Make 
DRAWINGS of the tubercles on pea, bean, clover and alfalfa. 



ETIOLOGY 

The cause of these tubercles is a bacterial pathogene, Bacterium 
leguminosarum (Frank) E. F. Smith. It is probable that this species 
includes a number of more or less distinct biologic forms. 

Life-history. This pathogene is a highly specialized parasite, develop- 
ing normally in living plant-tissues which it stimiilates to overgrowth. 
The galls which it induces are usually annual growths. It is probable 
that in most cases only primary cycles occur. 

The Primary Cycles are initiated in the spring and summer on the 
developing rootlets of the host. 

Pathogenesis. Active individuals in the soil-water constitute 
the inoculum. Study a mount from an actively growing pure culture 
provided . ob se rve : — 

4. The form and size of the bacteria. They are actively motile, 
having one polar flagellum (which can be seen only in specially stained 
preparations) . 

These bacteria in the soil accumulate in small groups on the sides of 
the root-hairs. By means of some secretion, they gradually soften the 
wall of the root-hair and penetrate it. The root-hair is stimulated to 
bend or curl about the mass of penetrating bacteria in a characteristic 
fashion. Study the prepared slides showing root-invasion, observe: — 

5. The curled tip and the infection-thread extending through 
the length of the root-hair. draw. 

6. Under the demonstration microscope an infection-thread 
within the tissue of the root. Note the funnel-lilce swellings produced 
just before the host-walls are penetrated, draw. 

These bacteria, having penetrated to meristematic cells, multiply 
rapidly and stimulate abnormal cell-division, resulting in the formation of 
the gall. 

170 



171 

Cut thin sections of tubercles. Examine the central bacterioidal region 
with the high-power, observe: — 

7. The numerous infection -threads extending from cell to cell. 
This shows how new cells, fonned by the primordial meristem, are infected 
from the adjoining bacteria-filled cells. 

8. The immense number of bacterioids in each invaded cell. 

9. That they are for the most part not rod-shaped like the indi- 
viduals in the soil, but are club-shaped to Y-shaped; most evident in the 
central cells. As the organisms in the host-cells or in old cultures cease 
to divide, they gradually take on these so-called involution-forms. (See 
Virginia Ann. rept. 1909-1910:136-137.) 

Some of the individuals in the outer cells of the bacterioidal tissue remain 
active and rod-shaped. The degenerate bacterioids are attacked by the 
enzymes of the host, and the nitrogen stored within them is made available 
for the growth and development of the host. 

Saprogenesis. With the death and disintegration of the tubercle- 
tissues at flowering-time of the host, the active rod-shaped bacteria in the 
gall are set free in the soil. Whether during their sojourn in the soil, 
they lead an active saprophytic existence, though highly probable, is not 
certain. They can be readily grown on artificial media. They are not 
known to produce spores. 

Pathological Histology. Longitudinal sections of tubercles and normal 
root -branches are provided. Examine and observe: — 

10. The general shape and appearance of the longitudinal 
section of the tubercle. Compare with that of the young lateral root. 

11. The kind of tissues from which the tubercles and the roots 
respectively arise. 

12. The vasctilar strands. Note their condition at the base 
of the bacterioidal region and the arrangement of the xylem and phloem. 
Compare with the condition in the young lateral root. 

13. The primordial meristem and root-cap. Are they present 
both in the tubercle and in the lateral root? 

14. The cortex. Is it continuous with that in the root? 

15. The epidermis. Is it present in the tubercle as well as in 
the lateral root ? 

16. The bacterioidal region. Where is it located in the tubercle? 
Note the size and contents of the cells. Note the outer cell-layer limiting 
the bacterioidal region, — the starch-sheath. 

What metaplastic conditions predominate in the tubercle, hypertrophy 
or hyperplasia? 

Make a diagrammatic drawing of a longitudinal section of a root- 
tubercle; similar comparative drawing of the lateral root. In these 
drawings only an outline need be made to indicate the comparative shape 
of the lateral root and the tubercle, and to locate the various tissues men- 
tioned above. 

Examine the prepared cross-sections of the tubercles, observe: — 

17. The bacterioidal region, ring of bundles, cortex and starch- 
sheath. DRAW. 



172 

IS. The effect of the bacterial invasion on the organs of the 
infected cells; nucleus,, cytoplasm and walls, as compared with those of a 
homologous healthy cell. (Read Fred, Virginia Ann. rept. 1909-1910:123, 
first paragraph.) Detail in enlarged drawings a healthy and an infected 
cell showing the cell-organs and contents. 

REPORT 

1. Present the evidence to show the pathological nature of the 
legume tubercles. Read in this connection: 

Smith, E. F. Bacteria in relation to plant diseases 2:97-146. 1911. 
Fred, E. B. The infection of root-hairs by means of Bacillus radicicola. 
Virginia Agr. Exp. Sta. Ann. rept. 1909-1910:123-137; and other 
references given in the text. 



PEACH LEAF-CURL 

This is a common and frequently destructive disease of peaches in 
nearly every peach-growing section. It is especially severe in regions 
subject to cold, wet weather in the spring. One of the most profitable 
commercial varieties, the Elberta, is particxdarly susceptible. 

SYMPTOMS 

It is chiefly a disease of the leaves, although the current season's shoots 
may also suffer. Shoot -infection is especially serious when nursery-stock 
is affected. Flowers and fruit rarely show lesions of the disease. 

On the leaves. Examine the material provided, observe: — 

1. The flexible character of the normal portions of the leaf. 
Tear it and note the texture. 

2. The wrinkled and distorted portion of the leaf affected by the 
curl. Tear and compare with the healthy portion as to texture and 
flexibility. Explain the difference. Compare with the healthy portion 
as to color and thickness. 

3. The character of the distortion. In which direction as re- 
gards the long axis is the diseased leaf wrinkled ? Why? DRAW. 

Examine photographs and Cornell Bui. 276, fig. 83-86. observe: — 

4. That the diseased leaves soon drop, partially or wholly 
defoliating the tree. Later new leaves are usuall}^ put forth. 

On the twigs. Study the specimens provided, observe: — 

5. The dwarfing of the shoot due to a shortening of the inter- 
nodes, giving it a rosette-like appearance. 

6. The much thickened fleshy stem, — hypertrophied ; more or 
less definite linear ridges often extending down on the previous years 
growth. 

7. The badly infected leaves. The leaves on diseased twigs 
are more severely affected than on the healthy twigs, for invasion becomes 
general or systemic and the leaf-fundaments become diseased as they 
are formed. Most of the affected shoots graduallv die. (Compare U. S. 
Agr. Dept., Veg. Phys. and Path. Div. Bui. 20, pi. V.) 

On flowers and fruit. If specimens or photographs are available, study 
the character of the lesions on these organs, draw. 

The curl on the leaves affects the fruit indirectly by preventing setting 
or by causing it to fall. (Study U. S. Agr. Dept., Veg. Phys. and Path. 
Div. Bui. 20, pi. XVII, and table 33, p. 124.) 

ETIOLOGY 

The cause of this curl is Exoascus deformans (Berkley) Fuckel, one 
of the primitive ascomycetes of the order Protodiscales. No definite 
fruit-body is formed, the asci developing directly from the mycelium in an 
exposed hymenium over the surface of the lesion. 

Life-history. Relatively little is positively known about the life- 
history of E. deformans. Opinions with respect to the life-habits of this 
pathogene are based very largely upon circumstantial evidence. No 
true asexual stage of the fungus is known. 

173 



174 

The Primary Cycles are initiated in the early spring when the buds 
swell and begin to open, by inoctilum which has overwintered on the hairy 
bud-scales. Presumably this inoculum is some kind of spores; just what 
kind and how they reach the bud-scales is not known. 

Pathogenesis. Examine the buds on the peach twigs provided. 
OBSERVE : — 

8. The hairy character of the outer bud-scales. Remove one 
and study it under low-power. Minute globose spores may sometimes 
be detected, lodged among the long hairs on the scale. These may belong 
to the leaf -curl pathogene, at least present evidence warrants the belief 
that such spores lodged in this way serve as the primary inoculum. Make 
an enlarged diagrammatic sketch of an infested bud. 

The spring rains which cause the buds to swell and to open, also cause 
the spores to germinate and send forth a slender germtube which enters 
the bud between the scales and penetrates the tender leaves. As the leaves 
develop, the mycelium within spreads between the cells and stimulates 
the growing tissue, resulting in a curling and fluting of the blade. The 
germtube may also penetrate into the growing tissues of the developing 
twig. 

Make thin sections through diseased leaves or twigs; clear in chloral 
hydrate ; wash and stain for some time in eosin or methyl blue ; wash and 
mount in water, locate : — 

9. The intercellular mycelium, fitting tightly in, and conforming 
with the intercellular spaces. No haustoria are formed, draw a bit of 
the mycelium with adjoining host-cells. 

Branches of this mycelium pass toward the surface between the epider- 
mal cells. Make thin tangential sections from the surface of the diseased 
leaf. Stain as above and study under the high-power, make out : — 

10. The very abundant subcuticular mycelium; its irregiilar 
septate form; its branching and anastomosing habit. From this mycelium 
arise the asci. draw. 

Study cross-sections (freehand or prepared) of a leaf through a lesion 
bearing asci . ob se rve : — 

11. The row of enlarged and angular epidermal cells of the host; 
their organs and contents. 

12. The layer of long, more or less angular asci, containing spores, 
standing at right angles on the epidermal cells. 

In the prepared slides, observe: — 

13. Some of the asci, small and young with deeply stained 
protoplasmic contents and nuclei. 

14. Mature asci containing deeply stained ascospores or empty; 
shape and size of the spores; number in an ascus. 

draw a portion of the epidermis showing asci, old and young, in dif- 
ferent stages of development. 

15. That these ascospores often increase in number within the 
ascus by budding like yeast (known by the large number of small spores 
which the asci contain), draw. 

Saprogenesis. The ascospores when mature are shot through a 
rupture in the top of the ascus. Their further history is unknown. The 
mycelium in a lesion produces but one crop of ascospores and then ap- 
parently dies with the leaf which soon falls to the ground. There is prob- 
ably some saprogenic activity on the part of this pathogene between the 



175 

dissemination of the ascospore in early summer and primary infection 
the following spring, but of this nothing is known. 

Secondary Cycles may or may not occur. This point is not deter- 
mined. The ascospores germinate in water by budding or by the pro- 
trusion of a short germtube when placed on leaves of the host. Artificial 
inoculation of the host has never resulted in infection. (Read U. S. Agr. 
Dep., Veg. Phys. and Path. Div. Bui. 20, bottom of 38 to 40.) copy 
figures from pi. IV, showing two types of spore-germination. 

Pathological Histology. Study the prepared cross-sections of a dis- 
eased leaf. Select a place in the section where the diseased tissues may be 
compared with the normal, observe: — 

16. The increased thickness of the leaf in the diseased portion. 
To what is this due, increase in size, or ntunber of cells (hypertrophy or 
hyperplasia) ? 

17. Which tissues are most affected, palisade or spongy paren- 
chyma ? 

18. The affect on the number, shape and arrangement of the 
palisade-cells; chloroplastids and cell-walls (hypoplasia and metaplasia). 
These cells were attacked before they began to differentiate into palisade- 
cells. 

19. The effect on the epidermal cells on the upper surface of the 
leaf; on the lower surface. 

Make a detailed drawing showing comparatively the histological 
structure of the diseased and the normal portions of the leaf. 

REPORT 

1. Write a letter to a peach-grower, giving brief and current 
directions for controlling peach leaf -curl. 



LEAF=BLISTER OF OAKS 

This is a very common leaf-disease of oaks. Frequent epiphytotics^ 
with more or less serious effects on the host, occur in the southern portion 
of the United States, while in the north very little damage is done. 

SYMPTOMS 

The leaves only are affected. In the dry material, note : — 

1. The blisters; more or less circular; varying greatly in size. 

2. That in some cases the spots have become confluent and the 
entire leaf is much curled. 

3. That the blister is usually convex on the upper surface of the 
leaf and concave on the lower; however, this is not a constant character. 

4. That the upper or convex surface of the blister is, in young 
spots, lighter green than the normal leaf-tissue but, that in the older spots, 
it becomes more or less flecked with brown areas. 

5. That the lower or concave surface of the blister is a bluish 
gray in the fresh condition but changes on drying to a dirty or brownish 
gray. The coloration and velvety appearance of the under surface is due 
to the fruiting stage of the causal fungus. 

Make a sketch to show the symptoms of leaf -blister. 

ETIOLOGY 

The pathogene causing the leaf -blister of oaks is Taphrina coerulescens 
(Desmazieres and Montague) Tiilasne, an ascomycetous fungus belonging 
to the order Protodiscales. No fruit-body is formed by these primitive 
ascomycetes. The asci are developed directly from the mycelium in a 
hymeniiun on the surface of the host. The peach leaf-curl pathogene, 
Exoascus deformans (Berkley) Fuckel, is closelv related to T. coerulescens 
(Desm. and Mont.) Tul. 

Life-history. As is the case with the peach leaf-curl fungus, the life- 
history of T. coerulescens is imperfectly known. Ascospores, borne on 
the blisters in the spring and summer, bud in the asci and form numerous 
secondary spores. These spores are supposed to hibernate in some way 
and initiate primary infections the following spring. 

The Primary Cycles are initiated in the early spring by the over- 
wintered inoculum (kind unknown). 

Pathogenesis. The pathogenic activities of this fungus are exhib- 
ited entirely by the sexual or ascigerous stage. In fact no true conidial 
form of this pathogene or, for that matter, of any of the entire order, 
the Protodiscales, to which it belongs, is known. 

Examine the stained cross-sections of a blister and note : — 

6. The mycelium. To what portion of the leaf is it confined? 

7. The layer of more or less angular asci standing at right angles 
to the lower epidermal cells. 

8. The wedge-shaped root-like projections pushed down between 
the lower epidermal cells and which serv^e as hold-fasts for the asci. 

9. The spores within the asci. They are small and either eight 
in ntunber or numerous (probably in some cases as many as a hundred). 
Some authors have suggested that the budding of the eight ascospores, 

176 



177 

which results in these numerous asexual spores without a mycelial stage 
intervening, constitutes in reality asexual reproduction, and that these 
spores are conidia. This is largely theoretical, however, to account some- 
how for a conidial stage. 

DRAW a portion of the lower epidermis as seen in prepared cross-sections 
showing mycelium, holdfasts, and young and old asci with spores. 

Sapr agenesis. The whereabouts and activities of this pathogene 
during saprogenesis, if such a period ensues, is unknown. 

Pathological Histology. Although the mycelium of T. coerulescens 
is confined to the space it makes for itself between the lower epidermal 
cells and the cuticle, the effect of the parasite is seen throughout the thick- 
ness of the leaf. In the stained cross-sections, find and compare the healthy 
with the diseased part of the leaf, note : — 

10. The hypertrophy in the lower epidermis, spongy mesophyl, 
palisade-mesophyl and upper epidermis. 

1 1 . The hyperplasia in the lower epidermis and spongy mesophyl- 
tissues. 

12. That in the normal tissue the cells of the lower epidermis 
are longer than broad. The effect of the fungus is to cause these cells to 
increase greatly in size and, being so tightly packed together, they elongate 
in a direction perpendicular to the surface of the leaf. Finally septa may 
be laid down at right angles to their long axes. 

13. That the cells of the spongy mesophyl are increased both 
in number and size and are tightly packed together, largely eliminating 
the intercellular spaces so common in the health}^ tissue. 

14. That the cells of the palisade-mesophyl are longer and wider 
than the normal. 

15. That the cells of the upper epidermis are somewhat larger 
and more globose than the normal. 

16. That the chloroplastids in the diseased cells are much 
smaller and fevN^er in number. 

Make drawings showing (comparatively) the histological structure 
of the diseased and normal portions of the oak leaf. Preserve correct 
proportions between the two drawings. 

REPORT 

1. From the literature on leaf -blister of oaks and the methods 
used for control of peach leaf-curl, suggest possible measures of control for 
leaf-blister. 



BLACK KNOT OF PLUMS AND CHEERIES 

This is one of the most striking diseases of stone-fruits. It is an indi- 
genous disease peculiar to our native wild species of plums and cherries 
and is frequently destructive to our cultivated forms, especiall}- certain 
varieties of plums and sour cherries. The disease is not known to occtir 
outside of North America. 

SYMPTOMS 

The disease affects only the woody parts of the host, usually only the 
twigs, though it may extend from spurs to the larger limbs or body of the 
tree. (See illustration specimen.) Two seasons are required for the 
full development of the knot . 

Study the material provided showing the character of the knots in the 
spring of the first season, observe: — 

1 . The relation of the knot to the twig. Is the entire circum- 
ference involved ? 

2. The shape and color of the gall. 

3. The relation of the swollen tissues to the epidennis and cork- 
la^'er of the healthy part. 

4. The character of the surface of the knot. The olivaceous 
lumps or patches scattered over the brov/n woody surface. These are 
the conidial stromata. These stromata bear mature conidia in the spring. 

Compare several specimens on the above points, sketch a typical 
knot. 

The galls are probably initiated at buds or short spurs. Study the 
very young galls provided and observe: — 

5. The unruptured swellings on the side of the spur or in the 
twig nearby. What does this indicate as to the infection-court? draw. 

Study some of the specimens showing the character of the galls the 
second season, observe: — 

6. The darker color and more hard and woody character of the 
galls. 

7. The continuous black perithecial stroma covering the exposed 
surface of the gall. 

S. The minute pimple-like perithecia arising from, and crowded 
over the surface of the stroma. 

9. Secondary galls arising just above or below some of the 
primary ones; known by their lighter color and conidial stromata. 

DRAW to show the character of secondary galls and secondary gall- 
fomiation. 

10. That affected twigs may be bent at right angles to the 
knotted side. draw. 

ETIOLOGY 

The black knot is caused by the ascomycetous fungus, Plowrigktia 
morbosa (vSchweinitz) Saccardo. 

Life-history. This pathogene presents in its life-history one striking 
variation from the usual type; it requires two years to complete a life-cycle. 
The Primary Cycles are initiated in the early spring by inoculiun 
produced on two-year-old knots. 

178 



179 

Pathogenesis. The ascospores constitute the inoculum for the 
primary cycles. With the scalpel remove a few of the mattire perithecia 
and crush in a drop of water or potassium hydroxide on a slide. Cover and 

OBSERVE : — 

11. The ascospores; their form, number in ascus, munber of 
cells and color, draw. 

If living material is available, study the germinating ascospores. 

These ascospores mature and are discharged from the perithecia, usually 
during March. They are carried to the buds on the previous year's growth 
and infect the twig. The development of the pathogene and the reaction 
of the host is slow. Slight swelling of the infected twig begin to show 
by autumn. It develops rapidly during the early days of the following 
spring and burst through the epidermis. On the exposed surface, the 
scattered conidial stromata develop on which conidiophores bearing conidia 
are matured in late spring or early smnmer. These conidia are dissemi- 
nated and initiate secondary cycles. 

Study cross-sections through the conidial stromata and observe: — 

12. The structure of the stroma; relation to mycelium in host; 
size of cells; color. 

13. The erect conidiophores; septate or non-septate? 

14. The conidia; shape, size, color and number of cells; where 
and how attached to conidiophores? 

Make a drawing to show these structures. 

After the cotiidia are matured and disseminated, the velvety covering 
of conidiophores gradually disappears, giving place to the continuous 
black crust-like stroma of the sexual stage which has been developing, 
so that by autumn the knot shows the black carbonaceous character, 
distinctive of its second season, and the minute pimple-Hke perithecia 
(fruit-bodies of the sexual stage) developing on the surface. 

During the first warm days of the spring (two 3^ears after primary 
infection) ascospores are formed within the asci in these perithecia. 

Examine with the hand-lens, the surface of mature galls provided. 
observe : — 

15. The minute black bodies crowded together over the surface; 
shape and size ; adherent to each other or separate on the stroma .■' 

16. The apex of the perithecia often depressed; a minute 
opening at the center of each, — the ostiolum. 

Study prepared sections through mature galls showing longitudinal sec- 
tions of perithecia. make out : — 

17. The relation of perithecia to each other and to the stroma; 
character of the stroma as compared with that of the conidial stage. 

18. The structure of the perithecial wall; ostiolum; relation 
of asci and paraphyses to the perithecial cavity. 

Crush in water one of the inature perithecia. observe: — 

19. The structure of the asci, ascospores and paraphyses. 
Make an enlarged drawing of a longitudinal section of the perithecium 

and supporting stroma to show these structures in detail. 

The ascospores mature in early spring and are shot, during rainy 
periods, from the asci out of the ostiolum. Caught by the breeze, they 
are carried to the trees and infection occurs in the swelling buds or develop- 
ing shoots. Thxis in two full years is the primary life-cycle, starting with 
ascospores, completed in the production of another crop of ascospores. 



180 

Saprogenesis. As the pathogene is continuously associated with 
its living host, it may be regarded as having no saprogenic phase in any of 
its life-cycles. However, as a matter of fact, there is often little li\'ing 
host -tissue in the galls during the period of the maturing of the ascospores. 
Secondary Cycles are initiated by the conidia, produced on the galls, 
during the next growing-season after that in which the primary infections 
were initiated. These secondary cycles require slightly less than two fidl 
years for their completion and, like the primary, end with the production 
of ascospores. The pathogene structures, developed during the secondary 
cycles, and the host reactions are like those of the primary cycles. 

Mycelium spreading from the primary (or secondary) galls into adja- 
cent healthy tissues often give rise to secondary galls. This is regarded by 
some as secondary infection (Stewart, Amer. Jour. Bot. 1:114). 

Pathological Histology. The effect of this pathogene on the host is one 
of stimulated overgrowth of the invaded tissues. Cut with a knife or scal- 
pel (not a razor) across one of the galls provided for this purpose. Trim 
the surface smooth and with hand-lens, make out: — 

20. The anatomical distortions that have occurred in the elements 
of the vascular cylinder and bark, comparing constantly with the struc- 
tures in the normal portion of the twig. Make a diagrammatic drawing 
of the entire cross-section showing normal and pathological anatomy. 
Label corresponding tissues in each. 

Study under the microscope the prepared sections through young one- 
year old galls, observe: — 

21. The character of the host -tissue on which lies the stroma 
of the fungus; size, shape, contents of cells and tissue-relations. 

22. The mycelium in the tissues; inter- or intracellular? 

23. The mycelial strands and fans in some places; their pseudo- 
parenchymatous character. What is their relation to the stroma? 

24. The depth toward the pith to which the pathological effects 
are evident. 

25. The broadening of the rays in the diseased region; due to 
hyperplasia or hypertrophy? 

26. The isolation of the cambium at the outer ends of the xylem- 
wedges; wanting in the abnormally broad rays. 

27. The inhibition of xylem-elements and the abnormal develop- 
ment of parenchyma in the xylem-wedges. The enlargement of the gall 
the second year is largely due to a marked hypertrophy of these parenchy- 
mal cells. 

28. The isolated groups of siunmer-wood vessels in the xylem- 
parenchyma just inside the cambitmi. 

29. The isolated xylem-elements in the diseased bark. These 
result from the misplaced and broken segments of the cam.bium opposite 
the broad rays. 

30. The proportional increase in thickness of bark and wood 
in the diseased region. The outer bark sluffs off before conidial formation. 

Make a full-page drawing, detailing cells in the different regions, to 
bring out the histological changes above emphasized. 

report 
1. Prepare a diagram showing the two types of life-cycles in 
Plowrightia morbosa (Schw.) Sacc. and their relation to each other. 



This is the most common and most noticeable disease of com. It is 
sometimes known as boil-smut to distinguish it from the head-smut. 
It affects all varieties of field-corn, pop-corn and sweet com. Some 
varieties of sweet com suffer severely from this smut. 

SYMPTOMS 

All parts of the plant abo\'e ground are subject to the disease. Unlike 
most of the cereal smuts, invasion is local. The lesions are characterized 
by marked hypertrophy. 

On the stalk. Study the specimen provided and observe: — 

1 . At what points on the stem the galls are usually located. 

2. The large size of the boils and the comparatively small area 
from which they arise. Is there any evidence of injury to the adjacent 
host-tissues ? 

3. The texture of the boils. They are more firm in fresh speci- 
mens, especially when yet immature ; the tough fleshy covering of such 
galls. 

4. The color; compare old and young boils. 

5. The dark dusty spore-mass filling the mature boils. 

A relati ^ely large portion of the hypertrophy is composed of pathogene 
structures (mycelium or spores). 

Make a sketch to show stem-boils. 

On the leaves. Lesions are comparatively rare on the leaves. In 
the specimens provided, observe: — 

6. The small boils arranged in rows parallel with the veins. 
Do they seem to arise from or between the veins; which side of the leaf? 

7. Any effect on the tissues adjacent to the boils; opposite 
the boils. 

sketch leaf -lesions. 

On the ear. Affected ears are very common, especially those with 
exposed tips. In the specimens provided, observe: — • 

8. The form, size and location of the boils. 

9. That the}' are each an enlarged kernel. Find kernels in 
different stages of hypertrophy. 

SKETCH a diseased ear to show healthy kernels and diseased ones, 
hypertrophied in various degrees. 

On the tassel. The boils of this smut are very common on tassels 
but may be confused with those of the head-smut. (See Kansas Bui. 
62:199 and pi. VI, VIII-X.) In the specimens provided, observe: — • 

10. That the boils on the tassels are but hypertrophied parts of 
the flowers. 

11. That only flowers here and there in the tassel are affected. 

12. That the character and degree of malformation varies in 
different flowers. 

Carefully dissect a healthy and a diseased flower. Make a series of 
comparative drawings to show the effect of the disease on different organs 
of the male flower. 

181 



182 

ETIOLOGY 

The pathogene causing com smut is Ustilago Zeae (Beckmann) linger, 
one of the Ustilaginaceae. It was first described as one of tlie puff-balls 
by Beckmann in 17GS and has been, since that time, a frequent subject 
for investigation by mycologists and phytopathologists. 

Life-history. It differs markedly in its life-habits from most of the com- 
mon cereal smut pathogenes. There are normally, in temperate climates 
at least, only the primary cycles. 

The Primary Cycles are initiated at any favorable time during the 
period of active growth of the host. Overwintered chlamydospores, 
usually in the manure on the field, are the sources of inoculum. 

Pathogenesis. Some of the chlamydospores from an old smut-boil 
have been germinated in manure-extract on the slides provided. Examine 
under the microscope and observe: — 

L3. The large globose spiny dark-brown chlamydospores. 

14. From some of these, the protruding hyaline promycehum 
(basidium). 

15. The crack in the epispore through which the promyceliiun 
is protruded. 

16. The septa in the promycelium, from below each of which 
arise one to several slender pointed sporidia. They are often produced 
in great numbers; sometimes in chains. 

DRAW a germinating chlam.ydospore showing abimdant sporidial 
production. These sporidia are carried by the wind to the growing com. 
SomiC of them lodge on the growing tissues of the host near the joints 
within the leaf-sheath, the emerging silks, or the blossoms on the tassels. 
Here they germinate, sending out germ tubes that penetrate these embry- 
onic tissues, and give rise to the mycelium. This mycelium does not spread 
through the tissues to any great distance from the point of infection; 
every boil probably arises from a separate infection. The hyphae grow 
in great masses into the hypertrophied tissues of the boil and form the 
greater bulk of the excrescence. Chlamydospores are soon developed in 
chains within short branches of this mycelium. The rest of the mycelium 
gelatinizes and disintegrates more or less. 

To study the mycelitmi in the host-tissue about the base of the boil, 
make thin longisections of the stalk, just beneath the boil. Stain with 
methyl blue or eosin. Examine and in the pith-tissues, observe: — 

17. The slender hyphae in the intercellular spaces or penetrating 
the cells; often in strands of several hyphae or gnarled in the intercellular 
spaces. 

18. Short knotted or irregiilarly swollen short branches sent 
into the cells, — the haustoria. 

DRAW to show the mycelium in the host-tissue. 

To study the distribution of the mycelium in the boil and the fonnation 
of the chlamydospores, examine prepared sections through a partly ma- 
tured boil. Make drawings to show the structures studied. 

Saprogenesis. Mount some of the black spore-mass in potassiiun 
hydroxide. Study with the high-power and observe: — 

19. The shape, size and color of the mature chlamydospores; 
the short spines which thickly beset the spore-wall. 



183 

20. Any variation in the spores as to size and shape. Look for 
bits of mycelium. 

Make an enlarged drawing of several of the spores. 

These chlamydospores go with the fodder into the manure and finally 
out on the land. They winter over in the manure and germinate 
during the following season. Their dependence upon saprophytic nourish- 
ment in germination is evidenced by the fact that they develop sporidia 
much better in a nutrient solution, e.g. manure-extract, than they do in 
water. 

REPORT 

1. Explain wh}^ seed treatments, so generally effective with smut 
diseases, will not control the corn smut. 



HOLLYHOCK RUST 

This is a rust disease peculiar to a number of malvaceous hosts. It is 
indigenous to ChiH where it was discovered in 1852. It is especially 
destructive to the hollyhock and is now known as a common disease of 
this perennial in nearly every temperate coimtry of the globe. 

SYMPTOMS 
The lesions of this disease occur on all the above-ground parts of the 
host, but are usually most numerous on the leaves. In severe cases, the 
calyx and seed-capsules are affected. 

On the leaves. Examine the specimens provided and observe : — 

1. The cushion -like pustules scattered over the leaf -surface, — 
sori of the pathogene; most abundant on which side? 

2. The variations in size and shape of the sori. 

3. Color. This varies with the age of the lesion. The yoimgest 
lesions usually show a distinct bright-yellow color, especially on the upper 
surface or where they occur on the veins and petioles. A little later the 
fully developed sorus (telium) takes on a rusty brown color and is sur- 
rounded by a yellowish zone. When the spores of the telium begin to 
germinate, the color becomes a grayish brown. As the pustule dies it 
turns black. 

4. The thickened character of the leaf-tissue in well developed 
lesions; there is slight hypertrophy. 

5. In old lesions, the dead and shriveled sorus surrounded 
by a narrow zone of necrotic leaf -tissue. This dead tissue sometimes falls 
out, leaving small round holes in the leaf-blade. 

DRAW a diseased leaf to show the characters of the lesions at various 
stages. 

Where the lesions are very numerous, the entire leaf-blade withers, 
turns brown and finally falls from the stem. The general effect of a 
severe infection is best observ^ed in the field. 

On the stems and the petioles. Examine the specimens provided. 

OBSERVE : — 

6. The shape and size of the pustules; usually larger than 
those on the leaves. Why? 

7. The elongated dead zone about the older lesions. How deep 
does the lesion extend? The lesions are very limited. They frequently 
dry out and disappear except for a large oval scar left b}^ the healed wound. 

DRAW several stem-lesions. 

On the fruit. Study the lesions on the flowers provided (best studied 
on the fruits of Malva rotundifolia L). observe: — 

8. The lesions on the calyx; compare with those on leaves and 
stems. 

9. The sori on the seed-carpels ; color, size and location, draw. 
The pathogene is often distributed on seed infested in this manner. 

ETIOLOGY 

The hollyhock rust is caused by a uredinaceous fimgus, Puccinia 
Malvacearum Montagne. 

184 



185 

Life-history. It is a so-called lepto-form or Leptopuccinia, i.e. it 
has neither aecia nor uredinia, producing only telia. The teliospores 
usually germinate in situ, as soon as mature, to form promycelia with 
sporidia. 

The Primary Cycles occur on the first leaves developed from the 
perennial root in the spring. The sources of inoculum are the old dead 
leaves, diseased the previous season, or overwintered living leaves in which 
the pathogene has hibernated. 

Pathogenesis. The inoculum consists of the sporidia. These 
are formed on the promyceliiun which develops from the teliospore. 
Teliospores which are produced late in the autumn often do not germinate 
at once biit remain dormant on the old dead leaves. More commonly, 
however, late infections on immature green leaves of the hollyhock or on 
those of Malva rotundifolia do not develop telia until early spring. The 
teliospores in these telia germinate and sporidia for the primary infections 
are formed. 

Study germinating teliospores; or illustrations in Phytopath. 1, pi. 
XIII. observe: — 

10. The long slender basidium (promyceliimi) , most commonly 
developed first from the apical cell. 

11. The densely granular protoplasm of the apical portion 
of the basidjum; its final division into four cells, and the formation of a 
sporidium from each. 

12. That sometimes the four apical cells separate before sporidia 
are developed. 

Make a series of drawings to show sporidial formation. 

The sporidia are carried by the wind or splashing rain to the leaves 
nearby. Here they quickly germinate sending forth a short germtube 
which penetrates the host. 

DRAW several germinating sporidia. These sporidia are uninuculeate 
and develop within the host-tissues a septate uninucleate myceliimi. 
Study thin sections (freehand or prepared) through a lesion, observe: — 

13. The slender, often matted, septate mycelium in the inter- 
cellular spaces or forcing the host-cells apart. Are haustoria formed? 

14. The matted mycelial stroma from which arise the slender 
teliospore-stalks, forming the telial sorus. 

15. The teliospores; form, size, color, septation and thickness 
of walls. 

16. Young spores in various stages of development. 

Make a detailed drawing of a telial sorus in section with adjacent 
host-tissue and intercellular mycelium. 

The teliospores germinate in situ, giving rise to the sporidia which, 
scattered by wind or rain, initiate secondary cycles. 

Secondary Cycles are initiated repeatedly throiighout the season 
and conform in their development to that of the primary cycles except 
that in the case of late secondary cycles, teliospores, which have developed, 
may fail to germinate at once and overwinter on dead leaves and stems. 
Very late secondary cycles on overwintered living leaves may fail to 
develop telia until the following spring. 



186 

Pathological Histology. Make a comparative study of diseased and 
healthy areas in prepared cross-sections of the leaf, determine : — 

17. What pathologic changes, if any, in number, form and size 
of cells has resulted from the presence of the pathogene. 

18. What changes have taken place in the protoplasts of affected 
cells; nucleus, cytoplasm and chloroplasts. 

Make drawings of diseased and healthy tissues to show the pathologic 
effects. 

REPORT 

1. Detail a plan for controlling the hollyhock rust in border- 
plantings. Give satisfactory reasons for each step in the procedure. 



RUST OF CEDAM AND APPLE 

This is a ver^^ coinmon disease of the apple in many sections of the 
country where the red cedar grows. It is especially prevalent in the Mis- 
sissippi Valley, northeastern New York, New England, Virginia, West 
Virginia and other southern states. The effect of the pathogene on both 
hosts is very marked. 

Most of the rust diseases of rosaceous plants are harbored by some 
species of cedar during the winter. In the case of the rust here con- 
sidered, the pathogene overwinters on the red cedar, Juniperus virginiana 
L. The Slimmer form of the pathogene occurs on wild crab, Pyrus 
coronaria L., as well as on the cultivated apple. The rusts appearing 
on pear, quince and hawthome are usually caused by species distinct 
from that causing the apple rust; all, however, are closely related. 

SYMPTOMS 

The symptoms differ strikingly on the two hosts. 
On the apple. The material provided was collected in midsummer. 
Examine the affected leaves and observe: — 

1. That the pathogene -produces a leaf -spot. How does it 
differ from, other leaf-spots? 

2. The differences in the character of the spot on the upper and 
lower surfaces as to color and definiteness of outline. 

3. The presence of black pimple-like structures on the upper 
surface, — the pycnia (spermagonia) of the pathogene, probably function- 
less male structures. 

4. The groups of brownish, somewhat cylindrical or fimbriate 
structures on the lower surface, — fruit-bodies called aecia (aecidia). 

SKETCH to show the above characters. 

The general effect of the pathogene is to give the trees a striking yel- 
lowish color in contrast to the dark green of the healthy trees nearby. 
Early defoliation follows severe infections. There is a marked variation 
in the susceptibility of varieties. Wealthy and Jonathan are especially 
susceptible. 

Lesions appear on the fruit when it is about one-fifth grown. They 
resemble those on the foliage. Examine diseased fruits provided and 
OBSERVE :— 

5. The color and location of the diseased area. 

6. The dwarfing effect on the fruit. Recall the effect on the 
leaf. 

7. The kinds of pathogene-structtires developed in these lesions. 
DRAW a diseased fruit. 

On the red cedar. The material provided was collected in early -spring 
(April) . OBSERVE : — 

8. The large brown galls. Form, size, consistency, and surface 
characters. 

9. The spore-cushions scattered over the surface, — the telia 
(teleuto-sori) . 

10. The attachment and the relation of the gall to the twig. 
Each gall results from an enlargement of a single leaf, according to certain 
authors or of the stem in the axil of the leaf, according to others. 

187 



188 

Remove one of the healthy leaves and note : — 

11. The size, form and relation to the twig. 

SKETCH a normal leaf and a diseased or galled leaf; maintain proper 
proportions. 

The most striking sign of this disease on the red cedar is exhibited during 
warm spring rains in April and May. From the telia. scattered over the 
gall, long yellow gelatinous teliospore-masses protrude, giving the effect, 
from a distance, of large yellow fruits; whence the popular names cedar- 
apples and cedar-flowers. 

ETIOLOGY 

This disease is caused by one of the Uredinales, Gymno sporangium 
Juniperi-virginianae Schweinitz. As already noted, the pycnial stage (O) 
and the aecial stage (I) occur on the leaves and fruits of the cultivated 
apple and wild crab. The telial stage (III) is found on the red cedar. 
A uredinial stage (II) is wanting. 

Life-history. The apple rust-fungus continues in close association 
with the living tissues of its hosts throughout its entire life-history. 
Primary Cycle 

Pathogensis. The inoculum for the primary cycle consists of 
sporidia which are produced on the red cedar. They are developed on a 
promycelium which in turn arises from a teliospore. Moisture of six or 
more hours duration is necessary for sporidial formation. As soon as 
the humidity decreases sufficiently to cause appreciable evaporation, 
the sporidia are forcibly ejected. This occurs from April to June. 
Sporidia are carried to the apple by the wind. Germination follows 
within a short time in the presence of moisture. Two types of germina- 
tion occur; one in which germtubes develop directly from the sporidia, 
and the other in which secondary sporidia are formed on a short germ tube. 
The latter type is more common. (See Phytopath. 3:282, fig. 1, and Ne- 
braska Rept. 22, pi. Ill, fig. 5; or Virginia Tech. btil. 9, fig. 9.) copy to 
show types of sporidial germination. 

The germtube arising from the secondary sporidium penetrates the 
cuticle. In the case of the leaf, this occurs on the upper surface. The 
mycelium developed from the sporidial germtube ramifies through a 
limited area of the leaf, with the result that tissue-changes are effected 
and certain fruit-bodies (O and I) are developed. Examine the apple 
leaves again, using hand-lens, observe: — 

12. The pycnia on the upper surface of the leaf; their form and 
distribution. 

13. The aecia on the lower surface of the leaf; their form, 
distribution and size. The long exerted peridium (aecial-wall), — a promi- 
nent character of aecia of this fungus. Rust-fungi showing aecia with 
such peridia belong to the form-genus, Roestelia. Accordingly the apple 
rust-fungus was formerly called Roestelia Pyrata (Schw.) Thaxter. 

Make enlarged drawings to show the pycnia and aecia as they appear 
under the hand-lens. 

Mount in potassium hydroxide some of the peridial cells and aeciospores 
from the aecium. Study and draw to distinguish them. 

Examine prepared slides showing sections through lesions on an apple 
leaf, observe : — 

14. The abnormal tissue-developments. Which tissues are 
involved? How affected? 



189 

15. The pycnia; their form and contents. 

16. The aecia; their position with reference to the pycnia, the 
relative size of the two structures; their form, structure and contents. 

Make a drawing to show the points observed in 14, 15 and 16. (See 
Virginia Tech. bul. 9, fig. 13.) 

The pycnial stage always precedes the aecial stage in the development 
of those rust -fungi possessing these spore-forms. Aeciospores never re- 
infect the apple. 

Secondary Cycles 

Pathogenesis. The inoculum for the secondary cycles consists 
of aeciospores which are discharged from July first to the end of the grow- 
ing-season. They are blown to the red cedar and there initiate the cedar- 
apples. The aeciospores apparently must undergo a period of rest before 
germination. The question of whether infection occurs in the fall or 
spring is yet unsettled. The way in which the germtube gets into the 
cedar is not known. The manner of germination is not essentially different 
from that of other fungous spores. A simple germtube is formed; this 
on entrance into the host-tissue develops into a mycelimn which stimulates 
the growing cells, and a gall results. This becomes evident the following 
June. Study the 5^oung green galls provided. These increase in size 
during the strmmer and become brown and ftdl sized by winter. The 
fungus passes the second winter of its life-history as mycelium in the full- 
sized gall. From the gall provided, remove a small bit of tissue to a drop 
of water on a slide and crush, observe : — 

17. The hyphal threads throughout the mount. The character 
of the host-cells. 

Examine prepared sections through mature galls, note : — 

18. That the gall shows cortex and vascular elements. 

19. The mycelium of the fungus and its relation to the host-cells. 
Are haustoria present? 

In early spring the mycelium forms, at certain places beneath the 
cortex, stromata from which are developed the telial horns. Examine 
galls showing these horns, note: — 

20. That the horns arise from galls as described. 

21. The depression at the base of each horn. 

22. The color, size and shape of the horns. What is their con- 
sistency when dry; when wet? 

On the end of each long stalk-cell (sporophore) a teliospore is devel- 
oped. These are matured by March and April. Germination of the 
teliospore results in the formation of a promycelium which bears sporidia. 
Make a mount of teliospores from the horns provided, observe: — 

23. The size, shape and color of the teliospores. 

24. Their manner of germination. (See Virginia Tech. bid. 
9, fig. 8, and Nebraska Rept. 22, pi. 3, fig. 5.) 

Make a drawing to show the points observed in paragraphs 17 to 24 
inclusive. (See Virginia Tech. bid. 9, fig. 5-8.) 

There is no saprogenesis in either the primar}^ or secondary cycles. 

report 

1. Discuss the relation of apple rust to environment. 

2. Summarize the practical value of such information. 



BLISTER-RUST OF WHITE PINE 

The blister-rust of five-needled pines and its alternate phase, the felt- 
rust of currants and gooseberries, has recently become an important 
disease in eastern United States. Previous to the importation of diseased 
white pine nursery-stock, this disease was confined to Europe. Recently 
it has assumed an epiphytotic character in several localities in northeastern 
United vStates. 

SYMPTOMS 

The symptoms produced by this disease will be studied in the order of 
their seasonal sequence on pine, currant and gooseberry. 

Blister-stage on pines. In the material and illustrations referred to, 
NOTE : — 

1. That the first evidence of the disease on the white pine is an 
indefinite, discolored canker-like area on the trunk or smaller branches. 

2. That in some cases a slight or marked hypertrophy of the 
bark occurs. (See PI. Ind. Bur. Bui. 206, fig. 3, 4 and pi. I; or Farmers' 
Bui. 742, fig. 3, 5 and pi. 1.) draav or copy. 

3. That often a diseased tree may be detected by its stunted 
growth and bushy appearance. (See Farmers' Bui. 742, fig. 1.) 

4. That the first external evidences of the pathogene are the 
minute light-yellow, bladder-like swellings which exude drops of liquid. 
These small pustules are pycnia (spermagonia) and within them are found 
minute spore-like bodies called pycnospores (spermatia). These spermatia 
are most generally believed to be non-functioning male gametes. They 
do not serve in any way to propagate the fungus. Usually the pycnial 
stage is formed several months after infection and during the autumn just 
previous to the production of aecia. 

5. The large hemispherical or pustular bladdery aecia pushed 
out from cracks in the invaded bark. draw. 

(i. The aecia, at first yellow and enclosed by a paper}^ peridium; 
later irregularly ruptured, allowing the yellow powdery contents to es- 
cape, — the aeciospores. The aecia are produced in early spring two or 
more years after infection. 

7. The pock-like depressions in the bark where aecia have been 
borne. These characteristic markings on the cankered area constitute a 
reliable means of identification of diseased trees at any time of the year 
after the first crop of aecia has been produced, draw. 

Summer stage on currants and gooseberries. Examine the diseased 
leaves of the different species of Ribes provided, observe : — 

8. That in the case of certain species definite leaf-spot lesions 
are produced; in others the fruit-bodies of the pathogene alone constitute 
the first signs of the disease, draw. 

9. The very small hemispherical pustiiles, — the uredinia, in 
groups on the under sides of the leaves. They are at first covered by the 
epidermis and are different in color from the remainder of the leaf, being 
slightly lighter green, draw as seen with the hand-lens. 

10. That later the uredinia become more or less orange-yellow in 
color; the covering (composed of leaf -epidermis and peridium) is rup- 
tured allowing the rusty yellow contents, — the uredospores, to escape. 

190 



191 

Felt-rust stage on currants and gooseberries. In the material of 
gooseberry and currant leaves provided, note : — 

11. The conspicuous hair-like growths, — the teha, often arising 
from the uredinia and more or less scattered over the entire under surface 
of the leaves, draw. In some cases the felt-rust stage accompanies 
the first uredina early in the summer but it is typically an autumnal stage, 
occurring in August and September. 

ETIOLOGY 

The blister-rust of five-needled pines is caused by the basidiomycetous 
rust-fungus, Cronartiuni ribicola Fischer von Waldheim. This name is 
based on the telial stage. The aecial stages of rusts, forming blister-like 
aecia on the bark or needles of conifers, have all been placed in the form- 
genus Peridermium. The blister-rust of five-needled pines is commonly 
known in its aecial stage as Peridermium strobi Klebahn. 

Life-history. This pathogene is a heteroecious fungus requiring two 
very different hosts for the development of its complete life-history. 
All the spore-stages prodticed by rtists are formed by this fungus: aecia 
(I) on bark of pines; uredinia (II), telia (III) and promycelia (IV) on 
the leaves of currants and gooseberries. The pycnia (O) are produced on 
the pine previous to the appearance of the aecia. 

Primary Cycles are initiated by the aeciospores produced in the 
aecia on the pine in early spring. When the peridiiun is ruptured, the 
aeciospores dust out and are blown about by the wind. 

Pathogenesis. The primary inoculiun, aeciospores from the pine, 
must reach young leaves of currants and gooseberries in order to function 
in continuing the development of the fungus. The aeciospores cannot 
infect the pine. 

Mount some of the yellow powder from the aecia in potassium, hydroxide, 
cover and examine with the microscope, observe : — 

12. The size, shape and color of the aeciospores; thickness and 
markings of the spore-wall. draw. 

The aeciospores, in the presence of moisture, germinate on the under 
sides of gooseberry and currant leaves and the germtubes penetrate into 
the stomatal cavities. After the establishment of a food-relation with the 
host-cells surrounding the sub-stomatal cavity, a mycelium is produced 
which spreads in a limited area. Within about two weeks after infection 
the uredinia are formed. In the material studied under symptoms of this 
stage, note: — 

13. The general character of the uredinia in different stages of 
development. 

14. The size, shape and color of the uredospores under the 
microscope. 

15. The thickness and markings of the spore-wall. 
DRAW a typical uredospore showing above points. 

In the prepared slides showing stained sections through uredinia, 
note : — 

16. The epidermis of the leaf which was raised up and finally 
ruptured by the developing uredinium. 

17. The peridimn enclosing the contents of the uredinitun. 



192 

18. The sporophores each bearing a single uredospore. How 
many nuclei in each spore? 

Make a drawing of the iiredinium in section as seen under the micro- 
scope. 

The first production of uredospores furnishes inoculum for the secondary 
cycles, which are repeated often during the summer on currants and goose- 
berries. 

In some cases the telial stage is produced from the samiC mycelium 
which produced the first crop of uredospores, and thus the primary cycle 
may be carried forward to the production of sporidia from the germinating 
teliospores. 

In the prepared slides showing uredinia in section, note : — 

19. The stroma developed just beneath the urediniiun and the 
radiating mycelivun extending into the host-tissiie. Find a haustorium. 

DRAW. 

In prepared slides showing the telial stage, observe: — 

20. That the telial horns arise directly from the same stroma 
that bore the uredospores. draw. 

21. The telial horn, composed entirely of teliospores cemented 
together into a compact mass. draw. 

22. The germination of the teliospores, forming a short germtube 
which is septate, — the promycelium. draw. 

23. The sporidia produced on sterigmata from each cell of the 
promycelium. 

The sporidia, produced as the last step in the primary cycle, initiate 
secondary cycles on the five-needled pines. The sporidia are formed during 
periods of wet weather. The teliospores germinate as soon as formed, 
there being no rest-period required. There is no saprogenesis in the life- 
history^ of this pathogene. 

Secondary Cycles. There are two sorts of secondary cycles in the 
case of this pathogene; those initiated by sporidia on pine, and those 
initiated by uredospores in currant and gooseberry leaves. The repeated 
succession of secondary cycles, initiated by uredospores, causes the rapid 
spread of the fungus from the currants and gooseberries harboring the 
primary cycles, until by the end of summer the fungus may be disseminated 
over several square miles. 

These secondary cycles on goosebeny and currant leaves duplicate 
the primary cycle, in that after the uredospores are dispersed, telial horns 
are produced from the same stromata and upon germination of the telio- 
spores, prom3"celia bearing sporidia are formed. The sporidia of the 
primary and secondary cycles are the inoculum which initiate the secondar^^ 
cycles on the five-needled pines. 

Very probably the sporidia are forcibly discharged from the sterigmata 
and they are blown to the pirie. They are short-lived and must find 
suitable conditions for germination soon or they will not function. The 
sporidia germinate by a germtube and this penetrates uninjured pine 
bark initiating a new mycelium. The steps in the development of the 
symptoms on pine have already been observed. Spermagonia may be 
formed in the autumn of the next year following infection. Aecia may 
be formed the second spring after infection or the production of the first 
crop of aecia may be delayed for several years. 



193 

From the material of affected pines, take a bit of the peridium; stain 
with eosin and mount in water, observe : — 

24. The character of the cells of the peridium. draw. 
Study the sections through white pine twigs bearing aecia. note : — 

25. The position of the aecia in relation to the tissues of the twig. 

26. The stroma at the base of the aecium and the radiating 
mycelium in the host-tissue. 

27. The origin of the peridium. 

28. The sporophores each bearing a chain of aeciospores. 
DRAW in detail an aecium with the surrounding host-tissues. 

REPORT 

1. Illustrate graphically the steps in the primary and secondary 
cycles of this pathogene. 

2. Illustrate graphically the nuclear phenomena exhibited during 
the life-history of this rust. 



MISTLETOE OF JUNIPERS 

The mistletoe of junipers is common in western and southwestern 
United States. A great deal of damage is annually caused by this and 
other mistletoe diseases of trees. 

SYMPTOMS 
In the juniper material provided , note : — 

1. The irregular and gnarled hypertrophies. 

2. The mistletoe plants firmly rooted in the host -tissue. 

3. That in many cases, the growth of the branches beyond 
the parasite is stopped. 

Make drawings showing the above symptoms. 

ETIOLOGY 

The parasite causing this disease of junipers is Phoradendron juniperi- 
num Englemann, one of the many species of the family Loranthaceae, a 
group of the flowering plants. This mistletoe has only very aborted 
leaves. They are mere scales closely appressed to the stem. 

Life-history. The inoculum in the case of the mistletoe is the small 
pulpy berry with its single inclosed seed. The berries are mature in 
Texas about December. Birds seek the berries for food and serve to a 
large degree as the disseminating agents. The stickiness of the pulp of 
the berry causes it to adhere closely to the branch of the host. 

In the material provided, observe: — 

4. The character of the berry, its pulpy flesh and the enclosed 
seed. DRAW. (See also bulletin mentioned below, fig. 1.) 

COPY PI. Ind. Bur. Bui. 166, fig. 2 and 3, showing the method of germina- 
tion of the mistletoe seed and the penetration of the host -tissue. 

The seedling establishes its root-like sinker in the conducting tissue of 
the host and obtains, not only water, but a certain amount of raw and 
modified plant-food from the host. The mature parasite blooms in 
December. The seeds require one year to develop and mature, so that 
they are ready for dissemination the next December. 

Pathological Histology. In the small branches of juniper cut longi- 
tudinally and transversely, study the roots of the mistletoe, observe : — 

5. The size, shape and number of roots sent from a single plant 
into the host -tissue. What tissues are invaded? draw. 

In the prepared cross-sections of juniper twigs, observe: — 

6. The penetration of the host-tissues by the mistletoe root. 

7. The elements present in the normal wood and bark. 

8. The elements present in the root of the mistletoe. 

9. The ultimate connection between the conductive elements 
of the host and of the parasite. 

Make drawings; (a) to show dia grammatically the relation of the 
tissues of the host and parasite, outlining the tissues; (b) to show in detail 
the ultimate connection of the conductive elements. 

report 
1. Outline a practical method for the control of the mistletoes 
in forest areas. Explain why it should be effective. Indicate the weak 
points in the program. 

194 



OEDEMA 

This disease, although seldom of much importance under field condi- 
tions, sometimes becomes destructive in the greenhouse. It is of interest 
to the plant pathologist in that it is not caused by an organism, but may 
readily be induced by changes in temperature and humidity or by the 
application of dilute solutions of certain toxic substances. It affects many 
plants but especially tomatoes and cabbage. 

SYMPTOMS 
The cabbage leaves provided were gathered from plants in a vegetable 
garden. The seedlings had been grown in the greenhouse, and the plants 
set out in the spring. The season was cold and rainy, observe : — 

1. That all the lesions are on the under side of the leaf. Is there 
any special reason why they should not be on the upper surface ? 

2. That these lesions are small raised spots, and are roughened 
like scab-spots on potatoes. Do they appear on the veins or on the areas 
between the veins? Is there any definite shape to the intumescences? 

Make a drawing of a portion of a diseased leaf. 

Study and sketch the symptoms as exhibited by potato, tomato or 
other plants provided, 

ETIOLOGY 

No organism is connected with this disease. It is caused by conditions 
which produce abnormal turgidity in the spongy parenchyma-cells of any 
part of the plant above the ground. It has been induced artificially by 
attaching the cut end of the stem to a hydrant where the water-pressure 
was very strong. The same result has also been accomplished by spray- 
ing the leaves with a dilute solution of ammoniacal copper carbonate. 
The larger drops always kill the tissue outright, but the smaller particles 
of spray cause an abnormal increase of the parenchymal cells below the 
epidermis. In the greenhouse, the disease results when the soil is warm, 
so that the roots take in a large amount of water, and the atmosphere 
is cold and the place poorly lighted, so that transpiration does not take 
place as fast as absorbtion. This causes such a pressure in the thin-walled 
tissvies that hypertrophy and rupture result. 

Under direction of the instructor, the students may undertake some 
experiments in the greenhouse to demonstrate the causal relation of some 
of the factors just mentioned. 

Pathological Histology. Cut thin cross-sections through a lesion or 
use prepared slides, observe: — 

3. That the lesion is made up of a number of abnormally large 
cells, the ends of which are entirely exposed. 

4. That this exposure is due to the rupturing and the breaking- 
away of the epidermal cells. Are fragments of these still remaining? 

5. That all the hypertrophied cells are those of the parenchyma. 

6. Changes in the organs of affected cells. 

Make drawings to show comparatively the conditions in diseased and 
healthy tissues. 

REPORT 

1. Describe in detail the procedure and results in the etiologic 
experiments, conducted under the instructor's directions. 

195 



TERM-PAPER SUBJECTS 

Each term-paper exercise will consist in preparing a short paper on 
one of the diseases listed. The paper shall be essentially a text on the 
disease chosen. 

INFORMATION 

1. The instructor will designate those subjects in the appended 
list from which selections for each term-paper are to be made. Any disease 
in the designated list may be chosen. The same subject may be chosen 
by more than one student, if approved by the instructor, but it is generally 
to the student's advantage to have as few working on the subject as pos- 
sible because of the limited number of available copies of some of the articles 
to be consulted. 

2. Students choosing the same subject will be expected to work 
independently in consisting the literature and preparing the paper. Any 
evidence of disregard of this expectation will rule a term-paper out of 
consideration. 

3. If so desired, the paper may be illustrated. This is not 
required, but will add to its value. Illustrations may be in the form of 
text-figures or plates. 

4. The student may write on some disease not listed herein, 
if permission is granted by the instructor. 

5. The selection of a subject for the first term -paper will be 
made at the beginning of the laboratory exercise on Literature of Plant 
Diseases. Selection of subjects for each of the other term-papers will be 
made later, at a time designated by the instructor. 

6. One laboratory period will be devoted to each term-paper. 
The work in the laboratory will consist of obtaining references and getting 
such information and assistance from the instructor as may be needed to 
make clear the method of procedure. 



PROCEDURE 

After having located and listed the references bearing on the subject 
chosen, as outlined in the exercise on Literature of Plant Diseases: — 

7. Select three or more of the most important articles (usually 
the most recent). 

8. Read and abstract these carefully according to directions 
given in the outline on Literature of Plant Diseases, p. 13-14. 

9. If specimens and materials of the disease chosen are available, 
they should be studied supplementary to the literature consulted, 

10. Spread out the abstracts and laboratory notes and proceed 
to correlate and arrange the data in each, according to the following 
outline : — 

196 



197 
OUTLINE FOR ARRANGING DATA IN TERM-PAPER 

THE BLACK EOT OF POMACEOUS FRUITS 

(References are to be arranged, in order of importance, thus: — ) 
Brooks, Charles, and DeMeritt, M. Apple leaf-spot. Phyto- 

path. 2:181-190, pi. 17, fig. 1-6. 1912. 
Paddock, W. The New York apple tree canker. New York 
(Geneva) Agr. Exp. Sta. Bui. 163:180-206, pi. I-VI. 

HOSTS 

PLANTS AFFECTED 
VARIETAL SUSCEPTIBILITY 

Of fruit to rot. 

Of foliage to leaf-spot. 

DISEASE 

NAMES 

HISTORY AND RANGE 

IMPORTANCE 

Nature of losses 

Injury to fruit 

Injury to foliage 

Reduction of yield 

Amount of losses 

SYMPTOMS 

On the fruit. 

On the leaves 

On the limbs 

ETIOLOGY 

Name, history and classification of the pathogene 

Pathogenicity 

Life-history. 



198 



The Primary Cycle 

Pathogenesis 

Inoculation 

Incubation 

Infection 

Sapr agenesis. 

The Secondary Cycles 

(If secondary cycles occur and require special considera- 
tion, repeat headings as for primary cycle.) 

ECOLOGY 

Pathological Histology. 

Influence of climatic factors 

Influence of soil factors 



Quarantine measures. 



Elimination. 
Cultivation. 

Rotation. 

Disinfection. 



CONTROL 

EXCLUSION 
ERADICATION 



PROTECTION 

Manipulation of normal environment 

Interfering with disseminating-a gents. 

Modifying sail-reactians 

Modifying temperature-relations 

Modifying moisture-relations 

Application of inhibiting substances 

Spraying 

Dusting 

Dressing and filling wounds 



IMMUNIZATION 

Selection 

Crossing 

Feeding 



199 

NOTES 

11. The heads and subheads appearing down the middle of the 
page are quite definitely fixed and will be the same for any subject chosen. 
They should appear, properly placed, in the manuscript submitted. 

12. The subheadings indicated along the left of the page will 
vary in character more or less, depending on the disease in hand, except 
in the case of those under etiology which will be uniformly the same for 
all diseases. 

13. Additional subheads of the type at the left of the page may 
be inserted where the nature of the data requires it. 

14. Omission of heads and subheads is to be made when 
there are no data to record thereunder. 

15. References in the body of the text should be inserted at the 
end of the sentence. 

Where one of the references in the list at the beginning of the papef 
is to be referred to, enclose in parenthesis the author's name, date of 
publication, colon and page, thus: — (Jones, 1914:27). The author's 
name may be omitted when it is clear, from the context, to which article 
reference is made. 

When an article not listed in the references given at the beginning of the 
paper, is to be cited, the parenthetical insertion must include the author's 
name, name of publication (abbreviated), volume or its equivalent, 
colon, and pages on which the data are to be found, thus: — (Duggar, 
Fungous Dis. p. 237. 1909) or (Peck, Joum. Myc. 7:10-14). 

16. Confine the discussion to the disease on the host or hosts 
specified and to closely related hosts. 

17. The reference-sheets, including the abstracts, must be handed 
in with the term paper. 

LIST OF SUBJECTS 

FIELD CROPS 

1. Alfalfa root-gall caused by Urophlyctis alfalfae Magnus. 

2. European root -rot of alfalfa caused by Rhizoctonia Crocorum Fries. 

3. Alfalfa stem-blight caused by Bacterium Medicaginis (Sackett) 
E. F. Smith. 

4. Smuts of barley caused by Ustilago Hordei (Persoon) Kellerman and 
Swingle, and Ustilago nuda (Jensen) Kellerman and Swingle. 

5. Clover anthracnose caused by Colletotrichuni Trifolii Bain ( = C 
caulivoruni Kirchner) . 

6. Clover rusts caused by Uromyces Trifolii (Hedwig) Leviell6 on white 
clover, and Uromyces fallens (Desmazieres) Kern on red clover. 

7. Stem-rot of clover caused by 5cferoff ma Tn/o/wMm Eriksson. 

8. Ear-rot of corn caused by Diplodia Zeae (Schweinitz) Ldvielle. 

9. Cotton anthracnose caused by Glomerella Gossypii (Southworth) 
Edgerton { = Colletotrichum Gossypii Southworth). 

10. Root-rot of cotton and alfalfa caused by Ozonium omnivorum Shear. 

11. Hekninthosporium diseases of cereals caused by Helminthosporium 
gramineum (Rabenhorst) Eriksson, H. teres Saccardo, H. sativum Pammel 
King and Bakke and H. turcinum Passerini. 



200 

12. Downy mildew of grasses caused by Sderospora graminicola (Sac- 
cardo) Schroeter. 

13. Hop mildew caused by Sphaerotheca Humuli (Fries) Burrill. 

14. Crown-rust of oats caused by Puccinia LolW^'ieison { = PMCcinia 
coronata Corda.) 

15. Early blight of potatoes caused by Alterania Solani (Ellis and 
Martin) Jones and Grout {=Macrosporium Solani Ellis and Martin). 

16. Leaf-roll and curly-dwarf of potatoes, cause unknown. 

17. Black leg of potatoes caused by Bacillus phytophthorus Appel 
( = Bacillus Solanisaprus Harrison, = Bacillus atrosepticus van Hall, = Bacil- 
lus melanogenes Pethybridge and Murphy). 

18. Potato scab caused by Actinomyces chromogenus Gasperini ( = Strep- 
tothrix scabies (Thaxter) Cunningham, =Oo5^ora scabies Thaxter). 

19. Black wart of potatoes caused by Chrysophlyctis endobiotica 
Schilbersky. 

20. Brown rot or wilt of Solanaceae (potatoes, tomatoes and tobacco) 
caused by Bacterium Solanacearum E. F. Smith {= Bacillus Solana- 
cearum E. F. Smith, =5aa7/w5 Nicotianae Ueda). 

2 1 . Rice blight or blast caused by Piricularia grisea (Cooke) Saccardo. 

22. Rice smut caused by Tilletia horrida Takahashi. 

23. Cobb's sugar-cane disease caused by Bacterium vascularum (Cobb) 
G. Smith. 

24. Root disease of sugar-cane caused by Marasmius plicatus Wakker. 

25. Smuts of sorghum caused by Sphacelotheca Sorghi (Link) Chnton, 
Ustilago cruenta Kiihn and Sphacelotheca Reiliana (Kiihn) Clinton. 

26. Timothy smut caused by Ustilago striae formis (West.) Niessel. 

27. Timothy rust caused by Puccinia Phlei-pratensis Eriksson and 
Henning. 

28. Root-rot of tobacco caused by Thielavia basicola (Berkley and 
Broome) Zopf. 

29. Wheat scab caused by Fusarium cumorum (W. G. Smith) Saccardo. 

30. Flag-smut of wheat caused by Urocystis Tritici Komicke and the 
stem-smut of r>^e caused by Urocystis occulata (Wallroth) Rabenhorst. 

31. Take-all of wheat caused by Ophiobolus graminis Saccardo or 
Ophiobolus herpotrichus (Fries) Saccardo. 

32. The role of insects in the dissemination of pathogenic fungi. 

FRUIT CROPS 

33. Fruit-spot of apple caused by Phoma Pomi Brooks { = Cylindro- 
sporium pomi Brooks). 

34. European apple tree canker caused by Nectria galligena Bresadola. 

35. BHster-canker of apple caused by Nummularia discreta (Schweinitz) 
Tulasne. 

36. Northwestern anthracnose of apple caused by Neofabrea malicorticis 
(Cordley) Jackson ( = Gloeosporium malicorticis Cordley) . 

;37. Apple rots caused by the following ascomycetous pathogenes; 
Alternaria sp., Cephalothecium roseum Corda., Volutella fructi Stevens and 
Hall, Endomyces Mali Lewis, Sclerotinia cinera (Bonorden) Schroeter. 

38. Water-core of apple, cause not definitely known. 

39. Crown-gall of apple and other fruit-trees caused by Bacterium 
tumefaciens E. F. Smith and Townsend. 



201 

40. Gummosi scaused by Bacterium Cerasus (Griffin) ( = Bacillus spongio- 
sus Aderhold and Ruhland). 

41. Leaf-blight (shot -hole or yellow-leaf) of cherries caused by Coc- 
comyces hiemalis Higgins ( = in part, Cylindrosporium Padi Karsten). 

42. Powdery mildew of cherry caused by Podosphaera Oxyacanthae 
(Fries) de Bary. 

43. Citrus canker caused by Bacterium Citri (Hasse) E. F. Smith 
( = Psendomonas Citri Hasse). 

44. Sooty mold of citrus caused by Meliola Penzigi Saccardo {=M. 
Cammeliae (Cattaneo) Saccardo). 

45. Citrus scab caused by Cladosporium Citri Massee. 

46. Wither-tip of citrus caused by Glomerella cingulata (Stoneman) 
Spaulding and von Schrenk ( = Colletotrichum gloeosporoides Penzig). 

47. Blue mold or rot of citrus caused by Penicillium digitatum (Fries) 
Saccardo, P. Italicum Wehmer and P. expansum Link. 

48. Brown rot and canker or gummosis of citrus caused by Pythiacystis 
Citrophthora E. H. Smith and R. E. Smith. 

49. Pod-rot and canker of cocoa caused by Phytophthora Faberi Mau- 
blanc. 

50. Leaf-spots of coffee caused by Pelliculiaria Koleroga Cooke, Stil- 
bella flavida (Cooke) Kohl, and other fungi. 

5 1 . An anthracnose of currants caused by Pseudopeziza Ribis (Libert) 
Klebahn { = Gloeosporium Ribis (Libert) Montagne and Desmazieres. 

52. The European currant rust caused by Cronartium Ribicola Fischer 
von Waldheim. 

53. Powdery mildew of gooseberries and currants caused by Sphaero- 
theca Mors-uvae (Schweinitz) Berkley and Curtis. 

54. Silver-leaf of fruit-trees caused by Stereum purpureum Fries. 

55. Root -rot of fruit trees caused by Armillaria mellea (Fries) Quelet 
and Clitocybe parasitica Wilcox. 

56. Injury to fruit-trees caused by frost. 

57. Grape anthracnose caused by Gloeosporium ampelophagum (Pas- 
serini) Saccardo {=Sphaceloma ampelinum de Bary). 

58. Dead arm or necrosis and ripe rot of grapes caused by Cryptosporella 
Viticola (Reddick) Shear (=Fusicoccum Viticolum Reddick). 

59. Powdery mildew of grapes caused by Uncinula necator (Schweinitz) 
Burrill. 

60. Olive knot caused by Bacterium Savastanoi E. F. Smith (= Bacillus 
olea-tuberculosis Savastano) . 

61. Black spot or bacterial shot-hole of peaches and plums caused by 
Bacterium Pruni E. F. Smith. 

62. California peach blight caused by Coryneum Beijrinckii Oudemans. 

63. Leaf -blight (shot-hole, yellow-leaf) of plimis caused by Coccomyces 
Prunophorae Higgins ( = in part, Cylindrosporium Padi Karsten). 

64. Die-back and canker of peaches and plums caused by Valsa leucos- 
toma Fries. 

65. Peach scab caused by Cladosporium carpophilum Thiimen. 

66. Pliim pocket caused by Exoascus Pruni Fuckel. 

67. Powdery mildew of the peach caused by Sphaerotheca pannosa 
(Fries) Levielle. 

68. Rust of stone-fruits caused by Puccinia Pruni-spinosae Persoon. 

69. Peach yellows, cause unknown. 



202 

yo. Little peach, cause unknown. 

7 1 . Leaf-spot of pear caused by Mycosphaerella sentina (Fries) Schroeter 
(=Septona Pyricola Desmazieres) . 

72. Pear scab caused by Venturia Pyrina Aderhold. 

73. Rust of the pear caused by Gymnosporangiuni glohosum Farlow, 

74. Leaf-bhght and fruit-spot of quince and pear caused by Fabrea 
maculata (Leveille) Atkinson {= Entomosporium maculatum Leveille), 

75. Rust of quince caused by Gymnosporangiwn clavipes Cooke and 
Peck. 

80. Anthracnose of raspberries and blackberries caused by Gloesoporium 
venetum Spegazzini. 

81. Orange-rust of raspberries and blackberries caused by Gymnoconia 
inter stitialis (Schlectendall) Lagerheim { = Gaeoma nitens (Schweinitz) 
Burrill). 

82. Cane-blight of raspberries caused by Leptosphaeria Coniothyrium- 
(Fuckel) Saccardo. 

83. Double blossom of dewberry caused by Fusarium Rubt Winter. 

84. Powdery mildew of strawberries caused by Sphaerotheca Humuli 
(Fries) Burrill. 

GARDEN CROPS 

85. Bean blight caused by Bacterium Phaseoli E. F. Smith. 

86. Downy mildew of lima-beans caused by Phytophthora Phaseoli 
Thaxter. 

87. Bean rust caused by Uromyces appendiculatus (Persoon) Levielld, 

88. Beet rust caused by Uromyces Betae (Persoon) Tulasne. 

89. Heart -rot and leaf-spot of beets caused by Phoma Betae Frank. 

90. Black leg and yellows or wilt of cabbage caused respectively by 
Phoma oleracea Saccardo and Fusarium conglutinans Wallenweber. 

91. The white rust of crucifers caused by Albugo Candida Kuntze 
( = Cystopus candidus Levielle). 

92. Early blight of celery caused by Cercospora Apii Fries. 

93. Leaf -spot and stem-rot of clematis caused by Ascochyta Clematidina 
Thumen. 

94. Sweet corn wilt or Stewart's sweet corn disease caused by Bacterium. 
Stewartii E. F. Smith. 

95. Scab of cucumbers caused by Cladosporium cucumerinum Ellis and 
Arthur. 

96. Wilt of cucurbits caused by Bacillus trachiephilus E. F. Smith. 

97. Angular leaf -spot of cucumbers caused by Bacterium lachrymans 
E. F. Smith and Bryan. 

98. Powdery mildew of curcurbits caused by Erysiphe cichoracearum 
Fries. 

99. Downy mildew of ginseng caused by Phytophthora cactorum (Cohn 
and Lebert) Schroeter. 

100. Alternaria blight of ginseng caused by Alternaria Panax Whetzel. 

10 1. Yellow disease of hyacinth caused by Bacterium hyacinthi (Wak- 
ker) E. F. Smith. 

102. Lilac blight caused by Phytophthora Syringae Klebahn. 

103. Blight of peas caused by Mycosphaerella pinodes (Berk, and Blox.) 
Stone { = Ascochyta pisi Libert) 

104. Root -rot of peas, beans, tobacco and ginseng caused by Thielavia 
basicola (Berkley and Broome) Zopf. 



203 

105. Dry-rot of sweet potatoes caused by Diaporthe Batatis Harter and 
Field { = Phoma Batatae Ellis and Halsted). 

106. Soft rot of sweet potatoes and leek of Irish potatoes caused by 
Rhizopus nigricans Ehrenberg. 

107. Stem- rot of sweet potatoes caused by Fusarium BatatatisWoWen- 
weber or F. hyperoxysporium Wollenweber. 

108. Scurf of sweet potatoes caused by Moiiilochaetes infuscans Halsted. 

109. Fruit-rot of tomato caused by Phoma destructiva Plowright. 
no. End-rot of tomatoes; cause variously assigned. 

111. Leaf-spot or blight of tomatoes caused by Sepioria Lycopersici 
Spegazzini. 

112. Leaf -mold of tomatoes caused by Cladosporium fulvuni Cooke. 

FOREST TREES 

113. Twig-blight of conifers caused by Pestalozzia funerea Desmazieres. 

114. Twig-bligh of conifers caused by Herpotrichia nigra Hartig. 
( = Neopeckia Coulteri (Peck) Saccardo) . 

115. Walnut blight caused by Bacterium Jnglandis (Pierce) E. F. 
Smith. 

116. Fir blight caused by Botrytis Douglasii Tubeuf. 

117. Leaf -blight of fir and spruce caused by Trichosphaeria parasitica 
Hartig { = Acanthostigma parasitica (Hartig) Saccardo). 

118. Tar-spot of maple leaves caused by Rhytisma acerinum Fries and 
R. punctatum Fries. 

119. Twig-blight of pine caused by Cenangium Ahietis Rehm. 

120. Leaf -spots of walnut and poplar caused respectively by Gnomonia 
leptostyla (Fries) Cesati and de Notaris ( = Marssonia Juglandis (Lebert) 
Saccardo) and Trochilia Populorum Desmazieres. 

121. Powdery mildew of chestnut caused by Phyllactinia Corylea 
Karsten. 

122. Powdery mildew of oaks in Europe caused by Microsphaera 
Alni (Fries) var. Quercina Neger. 

123. Canker of maple and other trees caused by Nectria cinnaharina 
Fries ( = Tubercularia vulgaris Fries) . 

124. Canker of spruce caused by Nectria cucurbitula Fries. 

125. Canker of spruce and fir caused by Pestalozzia Hartigii Tubeuf. 

126. Root-rot of conifers caused by Rhizina undulata Fries {=Rhizina 
inflata Quelet). 

127. Root-rots of trees caUvSed by Rosellinia sps. 

128. Root-rot of forest-trees caused by Armillaria mellea (Fries) 
Quelet. 

129. Red-rot of conifers caused by Fames Pinicola (Fries) Cooke. 

130. Decay of oak, beach and other trees caused by Bulgaria poly- 
morplia Wettstein ( = 5. inquinans Fries). 

131. Heart-rot of forest-trees caused by Poly par us sulphur eus Fries. 

132. Heart- and sap-rot of trees caused by Polystictus versicolor Fries. 

133. Compare and contrast symptoms produced by Polystictus versi- 
color Fries, Polystictus pergamenus Fries, Merulius lacrymans Fries and 
Fames Pinicola (Fries) Cooke. 

134. Compare and contrast symptoms produced by Polyporus squamo- 
sus Fries, Polyporus sulphureus Fries, Polyporus Betulinus Fries, Hydnum 
septentrionale Fries. 



204 

135- Compare and contrast symptoms produced by Trametes Pint 
Fries, Lenzites sepiaria Fries, Polyporus suhacidus Peck, Polyporus 
carneus Fries and Polyporus rimosus Berkley. 

136. Compare and contrast symptoms produced by Armillaria mellea 
(Fries) Quelet, Fomes annosus (Fries) Cooke, Polyporus Schweinitzii 
Fries and Thelephora galactina Fries. 

137. Compare and contrast symptoms produced by Fomes ignarius 
(Fries) Gillet, Fomes appalanatus Wallroth, Fomes Fraxanophilus (Peck) 
Saccardo Fomes fomentarius (Fries) Gillet. 

138. Host-index of wood-rotting fungi arranged under the names of 
the fungi. 

139. Host-index of wood-rotting fungi arranged under the names of the 
hosts. 

140. Leaf -burn of trees. 

141. Smoke and gas injury to trees. 

142. Winter injury to trees. 

143. Mycorrhiza of tree-roots. 



GLOSSARY* 

Agent of inoculation. — The thing which acts as the carrier in the transfer 
of the inoculum from its source to the infection-court, as for example : — 
wind, insects or running water. 

Control. — The prevention of losses from a disease. Every control measure 
is based on one of four fundamental principles; exclusion, eradication, 
protection or immunization. 

Dissemination. — The act or manner of scattering or spreading the inocu- 
lum of the pathogene within an immediate and more or less limited 
area about the source of inoculum. 

Distribution. — (a) The act of transporting and establishing a pathogene 
beyond barriers in other regions, (b) The geographical occurrence 
of the disease; synonymous with range. 

Ecology. — That phase in the study or discussion of a disease which deals 
with the relation of environmental factors to its occurrence, severity 
and character. The ecologic factors are chiefly, climatic, soil and 
cultural. They influence the disease indirectly through their in- 
fluence on the pathogene or the host or on both. 

Enphytotic. — The opposite of epiph}^otic. An enphytotic disease is one 
regularly occurring in a locality or region and not liable to marked 
variations in destructiveness. 

Epiphytotic. — The sudden and destructive appearance of a plant disease 
in a locality or region. An epiphytotic disease is one the past history 
of which shows it to have a tendency to appear suddenly and destruc- 
tively, usually over large areas at rather long intervals. The term 
is analogous in meaning to epidemic but not synonymous with it. 

Eradication. — The principle of controlling a plant disease by removing 
or destroying the pathogene already established within a given area 
or region. Disinfection, seed-selection, crop-rotation and the like 
are eradicatory measures. 

Etiology. — That phase in the study or discussion of a disease, which deals 
with the chief causal factor, the pathogene, its nature, character and 
relations with the host. 

Exclusion. — The principle of controlling a disease in plants by excluding 
the pathogene from a given area or region. Inspection and quaran- 
tine are the exclusionary measures usually employed. 

History of a disease. — The logically arranged historical facts, relating to 
the disease itself, as distinguished from those relating more es- 
pecially to the pathogene. 

Host. — The plant affected with, or subject to a given disease. 

H3rperplasia. — That type of pathological condition expressed by abnormal 
increase in the number of cells, that is, excessive cell-division. 

Hjrpertrophy. — That type of pathological condition expressed by abnormal 
increase in size of cells (dimensions or volume). The term is also 
commonly used in a less restricted sense to designate swellings or 
overgrowths of various kinds, due either to abnormal increase in 
the size of the cells or abnormal cell-division or both. (See p. 8.) 



*The definitions apply to the respective terms only as these terms are used in 
a phytopathological sense, and particularly as used in these outlines. The definitions 
do not always agree with those to be found in dictionaries, nor always with the variety 
of senses in which the terms are used even in phytopathological literature. 

205 



206 

Hypoplasia. — That type of pathological condition expressed by the failure 
of plant-cells or organs to complete, in one or more respects, their 
normal development, that is, arrested development. Dwarfing, 
failure of chlorophyl-development and the like, are examples. 

Hjrpoplastic diseases. — See p. 8. 

Immunization. — The principle of preventing losses from a plant disease 
by the development of resistant or immune strains of the crop. This 
may be accomplished by selection and propagation of naturally 
resistant or immune individuals, by segregation and propagation 
of resistant or immune individuals obtained by crossing immune 
and susceptible forms or, artificially by feeding or injecting into 
susceptible hosts, substances which will make them resistant or 
immune. The last has, as yet, little or no practical value. 

Incubation. — The activities and developments of the pathogene from the 
moment of its arrival in the infection-court tintil it has established a 
pathologic relation with the host. 

Incubation period. — The period beginning with the arrival of the inoculiun 
in the infection-court and ending with the first evidence of disease. 

Infect. — To initiate or produce disease. 

Infection.— The act of producing or initiating a diseased condition in the 
tissues of the host. Infection is progressive, developing cell by cell 
and continues as long as the host continues to react to the stimulus 
of the pathogene. 

Infection-court. — The place on or in the host where the incubation- 
activities of the pathogene take place; the immediate neighborhood 
of a possible point of infection. 

Infection period. — Commonly used to designate the period during which 
conditions (of host, pathogene, and environment) are especially 
favorable to inoculation, incubation, and initial infection, or to incuba- 
tion and initial infection only. 

Inoculate. — To transfer the inoculum from its source to the infection- 
court. 

Inoculation. — The act of inoculating. It includes all the phenomena in- 
volved in the transfer of the inoculum from its source to the infection- 
court. 

Inoculum. — That structure of the pathogene which may be transferred 
from its source to the infection-court. It usually consists of spores, 
seeds, eggs, thalli (of bacteria), or even mycelial fragments or pieces 
of stems (Cuscuta). 

Lesion. — A definite region in a plant or in one of its organs, characterized 
by a pathologic change in structure. 

Life-cycle. — The succession of phenomena exhibited during a period of 
continuous growth and development of the pathogene (in a fungus, 
from spore-germination to the normal death of the mycelium thus 
initiated). Most pathogenes exhibit in their life-cycles two rather 
distinct phases, pathogenesis and saprogenesis. 

Life-history. — The complete succession of phenomena characterizing a 
pathogene throughout the various cycles of its existence. 

Metaplasia. — That type of pathological condition expressed by the 
overgrowth in cells, other than increase in size (hypertrophy) or 
numbers (hyperplasia). Abnormal starch accumulation, abnormal 
development of chlorophyl, unusual thickening of cell-walls and 
abnonnal nuclear division or growth are evidences of metaplasia. 



207 

Sometimes used in a less restricted sense to include all types of over- 
growth. 

Metaplastic diseases. — See p. 8. 

Necrosis. — That type of pathological condition expressed by the rapid 
destruction of cell-structures and a consequent prompt death of 
the protoplasts; rots, blights and cankers are examples. 

Necrotic diseases. — See p. 7. 

Pathogene. — ^Any factor capable of initiating disease (usually a living 
organism). 

Pathogenesis. — That portion or phase of a life-cycle during which the 
pathogene becomes and continues directly associated with the living 
host. Pathogenesis includes inoculation, incubation and infection. 

Pathogenicity. — The ability of an organism to produce disease. 

Pathogenici^ studies. — Experimental studies demonstrating the patho- 
genicity of a given organism. 

Pathological anatomy. — That phase of phytopathology which deals with 
pathologic changes in form, appearance, arrangement and relation 
of tissues in plant-organs (gross internal symptoms). 

Pathological histology. — That phase of phytopathology which deals with 
pathologic changes in the individual cells of plant-tissues. 

Pathological morphology. — That phase of phytopathology which deals 
with pathologic changes in form, size, color and the like, of plants or 
plant-organs (gross or external symptoms). 

Primary cycle. — ^A life-cycle initiated by a primary infection. 

Primary infections. — Those infections first initiated by the pathogene 
after a period of rest or relative inactivity. In temperate regions, 
pathogenes usually initiate their primary infections in spring or early 
summer. 

Protection. — The principle of controlling a plant disease by placing some 
protective barrier between the host and the generally-present patho- 
gene. Spraying, dusting and coating with substances inimical to 
the inoculum of the pathogene but harmless to the host are the usual 
protective measures employed. 

Range. — The geographical regions, areas or countries in which the disease 
is known to occur. 

Saprogenesis. — That phase of a life-cycle during which the pathogene 
is not in direct association with the living host. Saprogenesis in- 
cludes the saprophytic activities and dormant period of the patho- 
gene. Some pathogenes exhibit no true saprophytic activities 
during saprogenesis; some exhibit no saprogenesis, being continu- 
ously associated with the living host. 

Secondary-cycle. — A life-cycle initiated b}' a secondary infection. 

Secondary infections. — Those initiated by inoculum from the primary 
or other secondary infections without an inteqaosed resting or dormant 
period. 

Symptoms. — Those pathologic changes by which a diseased plant is 
distinguished from a healthy one. For names and definitions of 
different kinds of symptoms, see pages 7-8. 

Signs. — Incidental or experimental evidences of disease as distinguished 
from pathological evidences. For names and definitions of some of the 
more usual signs of disease in plants, see pages 8-9. 

Source of inoculum. — The place or object on or in which the inoculum 
is produced. 



ilguls 



iffiSlSLP'' CONGRESS 




